Add spinner + better attribution in REPL. Several changes to the REPL,
 - better attribution of Emscripten (we talked about it but didn't do the change): the gif below doesn't include this change.
 - Attribute for the Jquery Terminal: I added a clickable grey link on the right top of the REPL. On mobile, I think it should be OK, since we don't really scale text currently, so there is still enough space to display it. There was an issue that this was the only text that was displayed while loading.  
 - So I ended up adding a spinner for loading as well. Potentially we could re-use it for indicating that a package is being loaded as it's continuously running in the background, we just need to put it in the foreground with `z-index: 2` and hide it again after the package is loaded.
 
![Peek 2022-05-28 12-05](https://user-images.githubusercontent.com/630936/170820858-7f2ad6dc-0eb5-4f4e-9920-b59cfe8044e9.gif)


from pathlib import Path
from typing import Any, Iterator

# TODO: support more complex types for validation

PACKAGE_CONFIG_SPEC: dict[str, dict[str, Any]] = {
    "package": {
        "name": str,
        "version": str,
        "_tag": str,
    },
    "source": {
        "url": str,
        "extract_dir": str,
        "path": str,
        "sha256": str,
        "patches": list,  # List[str]
        "extras": list,  # List[Tuple[str, str]],
    },
    "build": {
        "cflags": str,
        "cxxflags": str,
        "ldflags": str,
        "library": bool,
        "sharedlibrary": bool,
        "script": str,
        "post": str,
        "replace-libs": list,
        "unvendor-tests": bool,
        "cross-build-env": bool,
        "cross-build-files": list,  # list[str]
    },
    "requirements": {
        "run": list,  # List[str],
    },
    "test": {
        "imports": list,  # List[str]
    },
    "about": {
        "home": str,
        "PyPI": str,
        "summary": str,
        "license": str,
    },
}


def _check_config_keys(config: dict[str, Any]) -> Iterator[str]:
    # Check top level sections
    wrong_keys = set(config.keys()).difference(PACKAGE_CONFIG_SPEC.keys())
    if wrong_keys:
        yield (
            f"Found unknown sections {list(wrong_keys)}. Expected "
            f"sections are {list(PACKAGE_CONFIG_SPEC)}."
        )

    # Check subsections
    for section_key in config:
        if section_key not in PACKAGE_CONFIG_SPEC:
            # Don't check subsections if the main section is invalid
            continue
        actual_keys = set(config[section_key].keys())
        expected_keys = set(PACKAGE_CONFIG_SPEC[section_key].keys())

        wrong_keys = set(actual_keys).difference(expected_keys)
        if wrong_keys:
            yield (
                f"Found unknown keys "
                f"{[section_key + '/' + key for key in wrong_keys]}. "
                f"Expected keys are "
                f"{[section_key + '/' + key for key in expected_keys]}."
            )


def _check_config_types(config: dict[str, Any]) -> Iterator[str]:
    # Check value types
    for section_key, section in config.items():
        for subsection_key, value in section.items():
            try:
                expected_type = PACKAGE_CONFIG_SPEC[section_key][subsection_key]
            except KeyError:
                # Unknown key, which was already reported previously, don't
                # check types
                continue
            if not isinstance(value, expected_type):
                yield (
                    f"Wrong type for '{section_key}/{subsection_key}': "
                    f"expected {expected_type.__name__}, got {type(value).__name__}."
                )


def _check_config_source(config: dict[str, Any]) -> Iterator[str]:
    if "source" not in config:
        yield "Missing source section"
        return

    src_metadata = config["source"]
    patches = src_metadata.get("patches", [])
    extras = src_metadata.get("extras", [])

    in_tree = "path" in src_metadata
    from_url = "url" in src_metadata

    if not (in_tree or from_url):
        yield "Source section should have a 'url' or 'path' key"
        return

    if in_tree and from_url:
        yield "Source section should not have both a 'url' and a 'path' key"
        return

    if in_tree and (patches or extras):
        yield "If source is in tree, 'source/patches' and 'source/extras' keys are not allowed"

    if from_url:
        if "sha256" not in src_metadata:
            yield "If source is downloaded from url, it must have a 'source/sha256' hash."


def _check_config_build(config: dict[str, Any]) -> Iterator[str]:
    if "build" not in config:
        return
    build_metadata = config["build"]
    library = build_metadata.get("library", False)
    sharedlibrary = build_metadata.get("sharedlibrary", False)
    if not library and not sharedlibrary:
        return
    if library and sharedlibrary:
        yield "build/library and build/sharedlibrary cannot both be true."

    allowed_keys = {"library", "sharedlibrary", "script"}
    typ = "library" if library else "sharedlibrary"
    for key in build_metadata.keys():
        if key not in PACKAGE_CONFIG_SPEC["build"]:
            continue
        if key not in allowed_keys:
            yield f"If building a {typ}, 'build/{key}' key is not allowed."


def _check_config_wheel_build(config: dict[str, Any]) -> Iterator[str]:
    if "source" not in config:
        return
    src_metadata = config["source"]
    if "url" not in src_metadata:
        return
    if not src_metadata["url"].endswith(".whl"):
        return
    patches = src_metadata.get("patches", [])
    extras = src_metadata.get("extras", [])
    if patches or extras:
        yield "If source is a wheel, 'source/patches' and 'source/extras' keys are not allowed"
    if "build" not in config:
        return
    build_metadata = config["build"]
    allowed_keys = {"post", "unvendor-tests", "cross-build-env", "cross-build-files"}
    for key in build_metadata.keys():
        if key not in PACKAGE_CONFIG_SPEC["build"]:
            continue
        if key not in allowed_keys:
            yield f"If source is a wheel, 'build/{key}' key is not allowed"


def check_package_config_generate_errors(
    config: dict[str, Any],
) -> Iterator[str]:
    """Check the validity of a loaded meta.yaml file

    Currently the following checks are applied:
     -

    TODO:
     - check for mandatory fields

    Parameter
    ---------
    config
        loaded meta.yaml as a dict
    raise_errors
        if true raise errors, otherwise return the list of error messages.
    file_path
        optional meta.yaml file path. Only used for more explicit error output,
        when raise_errors = True.
    """
    yield from _check_config_keys(config)
    yield from _check_config_types(config)
    yield from _check_config_source(config)
    yield from _check_config_build(config)
    yield from _check_config_wheel_build(config)


def check_package_config(
    config: dict[str, Any], file_path: Path | str | None = None
) -> None:
    errors_msg = list(check_package_config_generate_errors(config))

    if errors_msg:
        if file_path is None:
            file_path = Path("meta.yaml")
        raise ValueError(
            f"{file_path} validation failed: \n  - " + "\n - ".join(errors_msg)
        )


def parse_package_config(path: Path | str, *, check: bool = True) -> dict[str, Any]:
    """Load a meta.yaml file

    Parameters
    ----------
    path
       path to the meta.yaml file
    check
       check the consistency of the config file

    Returns
    -------
    the loaded config as a Dict
    """
    # Import yaml here because pywasmcross needs to run in the built native
    # Python, which won't have PyYAML
    import yaml

    with open(path, "rb") as fd:
        config = yaml.safe_load(fd)

    if check:
        check_package_config(config, file_path=path)

    return config

from pathlib import Path
from typing import Any, Iterator

# TODO: support more complex types for validation

PACKAGE_CONFIG_SPEC: dict[str, dict[str, Any]] = {
    "package": {
        "name": str,
        "version": str,
        "_tag": str,
    },
    "source": {
        "url": str,
        "extract_dir": str,
        "path": str,
        "sha256": str,
        "patches": list,  # List[str]
        "extras": list,  # List[Tuple[str, str]],
    },
    "build": {
        "cflags": str,
        "cxxflags": str,
        "ldflags": str,
        "library": bool,
        "sharedlibrary": bool,
        "script": str,
        "post": str,
        "replace-libs": list,
        "unvendor-tests": bool,
        "cross-build-env": bool,
        "cross-build-files": list,  # list[str]
    },
    "requirements": {
        "run": list,  # List[str],
    },
    "test": {
        "imports": list,  # List[str]
    },
    "about": {
        "home": str,
        "PyPI": str,
        "summary": str,
        "license": str,
    },
}


def _check_config_keys(config: dict[str, Any]) -> Iterator[str]:
    # Check top level sections
    wrong_keys = set(config.keys()).difference(PACKAGE_CONFIG_SPEC.keys())
    if wrong_keys:
        yield (
            f"Found unknown sections {list(wrong_keys)}. Expected "
            f"sections are {list(PACKAGE_CONFIG_SPEC)}."
        )

    # Check subsections
    for section_key in config:
        if section_key not in PACKAGE_CONFIG_SPEC:
            # Don't check subsections if the main section is invalid
            continue
        actual_keys = set(config[section_key].keys())
        expected_keys = set(PACKAGE_CONFIG_SPEC[section_key].keys())

        wrong_keys = set(actual_keys).difference(expected_keys)
        if wrong_keys:
            yield (
                f"Found unknown keys "
                f"{[section_key + '/' + key for key in wrong_keys]}. "
                f"Expected keys are "
                f"{[section_key + '/' + key for key in expected_keys]}."
            )


def _check_config_types(config: dict[str, Any]) -> Iterator[str]:
    # Check value types
    for section_key, section in config.items():
        for subsection_key, value in section.items():
            try:
                expected_type = PACKAGE_CONFIG_SPEC[section_key][subsection_key]
            except KeyError:
                # Unknown key, which was already reported previously, don't
                # check types
                continue
            if not isinstance(value, expected_type):
                yield (
                    f"Wrong type for '{section_key}/{subsection_key}': "
                    f"expected {expected_type.__name__}, got {type(value).__name__}."
                )


def _check_config_source(config: dict[str, Any]) -> Iterator[str]:
    if "source" not in config:
        yield "Missing source section"
        return

    src_metadata = config["source"]
    patches = src_metadata.get("patches", [])
    extras = src_metadata.get("extras", [])

    in_tree = "path" in src_metadata
    from_url = "url" in src_metadata

    if not (in_tree or from_url):
        yield "Source section should have a 'url' or 'path' key"
        return

    if in_tree and from_url:
        yield "Source section should not have both a 'url' and a 'path' key"
        return

    if in_tree and (patches or extras):
        yield "If source is in tree, 'source/patches' and 'source/extras' keys are not allowed"

    if from_url:
        if "sha256" not in src_metadata:
            yield "If source is downloaded from url, it must have a 'source/sha256' hash."


def _check_config_build(config: dict[str, Any]) -> Iterator[str]:
    if "build" not in config:
        return
    build_metadata = config["build"]
    library = build_metadata.get("library", False)
    sharedlibrary = build_metadata.get("sharedlibrary", False)
    if not library and not sharedlibrary:
        return
    if library and sharedlibrary:
        yield "build/library and build/sharedlibrary cannot both be true."

    allowed_keys = {"library", "sharedlibrary", "script"}
    typ = "library" if library else "sharedlibrary"
    for key in build_metadata.keys():
        if key not in PACKAGE_CONFIG_SPEC["build"]:
            continue
        if key not in allowed_keys:
            yield f"If building a {typ}, 'build/{key}' key is not allowed."


def _check_config_wheel_build(config: dict[str, Any]) -> Iterator[str]:
    if "source" not in config:
        return
    src_metadata = config["source"]
    if "url" not in src_metadata:
        return
    if not src_metadata["url"].endswith(".whl"):
        return
    patches = src_metadata.get("patches", [])
    extras = src_metadata.get("extras", [])
    if patches or extras:
        yield "If source is a wheel, 'source/patches' and 'source/extras' keys are not allowed"
    if "build" not in config:
        return
    build_metadata = config["build"]
    allowed_keys = {"post", "unvendor-tests", "cross-build-env", "cross-build-files"}
    for key in build_metadata.keys():
        if key not in PACKAGE_CONFIG_SPEC["build"]:
            continue
        if key not in allowed_keys:
            yield f"If source is a wheel, 'build/{key}' key is not allowed"


def check_package_config_generate_errors(
    config: dict[str, Any],
) -> Iterator[str]:
    """Check the validity of a loaded meta.yaml file

    Currently the following checks are applied:
     -

    TODO:
     - check for mandatory fields

    Parameter
    ---------
    config
        loaded meta.yaml as a dict
    raise_errors
        if true raise errors, otherwise return the list of error messages.
    file_path
        optional meta.yaml file path. Only used for more explicit error output,
        when raise_errors = True.
    """
    yield from _check_config_keys(config)
    yield from _check_config_types(config)
    yield from _check_config_source(config)
    yield from _check_config_build(config)
    yield from _check_config_wheel_build(config)


def check_package_config(
    config: dict[str, Any], file_path: Path | str | None = None
) -> None:
    errors_msg = list(check_package_config_generate_errors(config))

    if errors_msg:
        if file_path is None:
            file_path = Path("meta.yaml")
        raise ValueError(
            f"{file_path} validation failed: \n  - " + "\n - ".join(errors_msg)
        )


def parse_package_config(path: Path | str, *, check: bool = True) -> dict[str, Any]:
    """Load a meta.yaml file

    Parameters
    ----------
    path
       path to the meta.yaml file
    check
       check the consistency of the config file

    Returns
    -------
    the loaded config as a Dict
    """
    # Import yaml here because pywasmcross needs to run in the built native
    # Python, which won't have PyYAML
    import yaml

    with open(path, "rb") as fd:
        config = yaml.safe_load(fd)

    if check:
        check_package_config(config, file_path=path)

    return config
