Statically alias distutils-stubs to setuptools._distutils

[Avasam]

Summary of changes

Closes https://github.com/pypa/setuptools/issues/4689 This is an alternative to https://github.com/pypa/setuptools/pull/4691. This makes type-checkers and editors see distutils as basically an alias to setuptools.

Advantage:

• Less maintenance work, these stubs can be autogenerated when updating the vendored stubs (if a file is removed or added)
• No need to keep in sync with typeshed
• Static typing follows directly pypa/distutils without concerns with stdlib/distutils compatibility

Disadvantage:

• Typing isn't as good or as complete as typeshed's (solved by https://github.com/pypa/distutils/pull/329)
• Typing improvements and fixes need to be done "in source" in pypa/distutils. But this is somewhat already the long-term plan (https://github.com/pypa/setuptools/issues/2345#issuecomment-1627561833 but directly in pypa/distutils)
• pypa/distutils could accidentally introduce a typing breaking change for setuptools, which probably needs to be ignored with a TODO when updating the vendor.

Pull Request Checklist

• [x] Changes have tests (existing type-checking tests)
• [ ] News fragment added in newsfragments/. (no need until this is shipped with setuptools) (See documentation for details)

[Avasam]

I've added a comment at the top of geenrated files to make it clear that they are generated and how to regenerate them.

[Avasam]

I'm currently putting my efforts towards https://github.com/pypa/setuptools/pull/4861 as a solution. Which does the same thing, but autogenerates the stubs on install/wheel creation instead of having them being checked in.