Statically alias distutils-stubs to setuptools._distutils

[Avasam]

Summary of changes

Closes #4689
This is an alternative to #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
• Typing improvements and fixes need to be done "in source" in pypa/distutils. But this is somewhat already the long-term plan (Add type hints to setuptools #2345 (comment) 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

• 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.