Packages the distutils stdlib module as a standalone package,
```
377K distutils.data
221K distutils.data.br
19K distutils.js
```
to reduce the minimal pyodide size.

Part of #1542 

This also,
 - adds documentation on what's supported or not in the stdlib
 - makes a few changes to `pyodide_build/buildall.py` to be able to add such stdlib packages as a dependency in other packages (which didn't work before). For instance, until there is a release with https://github.com/pypa/packaging/pull/396 micropip has a dependency on distutils.
Unvendor distutils. Packages the distutils stdlib module as a standalone package,
```
377K distutils.data
221K distutils.data.br
19K distutils.js
```
to reduce the minimal pyodide size.

Part of #1542 

This also,
 - adds documentation on what's supported or not in the stdlib
 - makes a few changes to `pyodide_build/buildall.py` to be able to add such stdlib packages as a dependency in other packages (which didn't work before). For instance, until there is a release with https://github.com/pypa/packaging/pull/396 micropip has a dependency on distutils.
(debugging)=
# Interactive Debugging
See [Emscripten's page about
debugging](https://emscripten.org/docs/porting/Debugging.html) which has
extensive info about the various debugging options available.

[Chromium has support for DWARF
info](https://developer.chrome.com/blog/webassembly/) which can be very helpful
for debugging in certain circumstances. To build Pyodide with DWARF, you should
set `EXTRA_CFLAGS="-g4"` and `EXTRA_LD_FLAGS="-g4 --source-map-base
"http://localhost:<port>/"` (substitute in your favorite port). Make sure to
rebuild CPython with these flags set (it isn't necessary to rebuild emsdk).

Once you have done this, when you load Pyodide, you will see 404 errors for
requests for the various source maps. In order to load the source maps
correctly, you will need to run a custom server that "fixes" the source map
urls. The following debugging server seems to work for both CPython and numpy.
Run it in the Pyodide root directory. If you need to debug other C extensions,
you will need to update the server. It should be clear what to do based by
looking at the 404s generated by the server and locating those files in the file
tree, perhaps by using `find`.
```py
import socket
import socketserver
from http.server import SimpleHTTPRequestHandler
import pathlib

alternative_bases=["cpython/build/Python-3.8.2/","src/", "build/"]
def fixup_url(path):
    if pathlib.Path("." + path).exists():
        return path
    for base in alternative_bases:
        q = pathlib.Path(base + path)
        if q.exists():
            return str(q)
    # Numpy source maps can be in a bunch of different places inside of the
    # directory tree, so we need to glob for them.
    dir = list(
        pathlib.Path("packages/numpy/build/numpy-1.17.5/").glob("**" + path)
    )
    if dir:
        return str(dir[0])
    return path


class MyTCPServer(socketserver.TCPServer):
    def server_bind(self):
        """Use socket.SO_REUSEADDR to allow faster restart"""
        self.socket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
        self.socket.bind(self.server_address)

class Handler(SimpleHTTPRequestHandler):
    def end_headers(self):
        # Enable Cross-Origin Resource Sharing (CORS)
        self.send_header('Access-Control-Allow-Origin', '*')
        super().end_headers()

    def do_GET(self):
        self.path = fixup_url(self.path)
        super().do_GET()


if __name__ == '__main__':
    port = 8000
    with MyTCPServer(("", port), Handler) as httpd:
        print("Serving at: http://127.0.0.1:{}".format(port))
        httpd.serve_forever()
```

(debugging)=
# Interactive Debugging
See [Emscripten's page about
debugging](https://emscripten.org/docs/porting/Debugging.html) which has
extensive info about the various debugging options available.

[Chromium has support for DWARF
info](https://developer.chrome.com/blog/webassembly/) which can be very helpful
for debugging in certain circumstances. To build Pyodide with DWARF, you should
set `EXTRA_CFLAGS="-g4"` and `EXTRA_LD_FLAGS="-g4 --source-map-base
"http://localhost:<port>/"` (substitute in your favorite port). Make sure to
rebuild CPython with these flags set (it isn't necessary to rebuild emsdk).

Once you have done this, when you load Pyodide, you will see 404 errors for
requests for the various source maps. In order to load the source maps
correctly, you will need to run a custom server that "fixes" the source map
urls. The following debugging server seems to work for both CPython and numpy.
Run it in the Pyodide root directory. If you need to debug other C extensions,
you will need to update the server. It should be clear what to do based by
looking at the 404s generated by the server and locating those files in the file
tree, perhaps by using `find`.
```py
import socket
import socketserver
from http.server import SimpleHTTPRequestHandler
import pathlib

alternative_bases=["cpython/build/Python-3.8.2/","src/", "build/"]
def fixup_url(path):
    if pathlib.Path("." + path).exists():
        return path
    for base in alternative_bases:
        q = pathlib.Path(base + path)
        if q.exists():
            return str(q)
    # Numpy source maps can be in a bunch of different places inside of the
    # directory tree, so we need to glob for them.
    dir = list(
        pathlib.Path("packages/numpy/build/numpy-1.17.5/").glob("**" + path)
    )
    if dir:
        return str(dir[0])
    return path


class MyTCPServer(socketserver.TCPServer):
    def server_bind(self):
        """Use socket.SO_REUSEADDR to allow faster restart"""
        self.socket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
        self.socket.bind(self.server_address)

class Handler(SimpleHTTPRequestHandler):
    def end_headers(self):
        # Enable Cross-Origin Resource Sharing (CORS)
        self.send_header('Access-Control-Allow-Origin', '*')
        super().end_headers()

    def do_GET(self):
        self.path = fixup_url(self.path)
        super().do_GET()


if __name__ == '__main__':
    port = 8000
    with MyTCPServer(("", port), Handler) as httpd:
        print("Serving at: http://127.0.0.1:{}".format(port))
        httpd.serve_forever()
```
