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.
--- a/skimage/io/manage_plugins.py  2019-03-19 07:17:05.000000000 +0100
+++ b/skimage/io/manage_plugins.py	2020-12-16 12:03:52.000000000 +0100
@@ -41,7 +41,7 @@
 # the following preferences.
 preferred_plugins = {
     # Default plugins for all types (overridden by specific types below).
-    'all': ['imageio', 'pil', 'matplotlib', 'qt'],
+    'all': ['imageio', 'pil', 'matplotlib'],
     'imshow': ['matplotlib'],
     'imshow_collection': ['matplotlib']
 }

--- a/skimage/io/manage_plugins.py  2019-03-19 07:17:05.000000000 +0100
+++ b/skimage/io/manage_plugins.py	2020-12-16 12:03:52.000000000 +0100
@@ -41,7 +41,7 @@
 # the following preferences.
 preferred_plugins = {
     # Default plugins for all types (overridden by specific types below).
-    'all': ['imageio', 'pil', 'matplotlib', 'qt'],
+    'all': ['imageio', 'pil', 'matplotlib'],
     'imshow': ['matplotlib'],
     'imshow_collection': ['matplotlib']
 }
