Generate distutils-stubs on install

[Avasam]

Summary of changes

3rd iteration to close #4689 (doesn't solve everything mentioned in that issue, but it would greatly reduce the scope so I'd prefer re-opening as a new issue)

This is built off of #4704 where the stubs are automatically generated, but now they're done on install instead of existing in the source code.

Further Advantages over #4704:

• Doesn't pollute the source or history

Further Disadvantages over #4704:

• Contributors must install the local setuptools repo in their environment (pip install .) to get proper type-checking in their editor on Python 3.12+ when it comes to pypa/distutils derived types. (they already had to install types-setuptools, this replaces that)

Pull Request Checklist

• Changes have tests (existing build+install and type-checking tests)
• News fragment added in newsfragments/.
  (See documentation for details)

[Avasam]

Aaaah, right, I forgot that *-stubs will never override the stdlib types from typeshed. So this only affects Python 3.12+
For end users: This is already the case with typeshed's own setuptools stubs, so that's fine. (This is true even of #4689)
For setuptools contributors: This means we can't use this to get correct types in our own code, when pypa differs from stdlib on Python < 3.12

Overall still an improvement (and typeshed won't have to special case this stub anymore) and a necessary step for #2345

[Avasam]

Alternative code for readability (but less "correct" because replace is run on an unnecessary extra bit of str):

Suggested change

	module = "setuptools._distutils." + str(relative_path.with_suffix("")).replace(
	os.sep, "."
	).removesuffix(".__init__")
	module = (
	("setuptools._distutils." + str(relative_path.with_suffix("")))
	.replace(os.sep, ".")
	.removesuffix(".__init__")
	)

Playing with it some more (also changed with_suffix for removesuffix, just to see):

Suggested change

	module = "setuptools._distutils." + str(relative_path.with_suffix("")).replace(
	os.sep, "."
	).removesuffix(".__init__")
	module = (
	"setuptools._distutils." # (don't unwrap)
	+ str(relative_path)
	.removesuffix(".py")
	.replace(os.sep, ".")
	.removesuffix(".__init__")
	)

[Avasam]

@jaraco I think that this version has minimal (if any) downsides, solves real issues (addresses the user-facing concerns of #4689 and part of the setuptools internal typing ones), is literally as good as typeshed's and works towards #2345

[Avasam]

I think this doesn't work with tox because of PEP660 editable installs? I'll need help to get that working.

[Avasam]

I very nearly got it working with a custom build backend to support PEP 660 editable installs, which also happens to leave the stubs in the source (which I can just .gitignore). Bringing all the benefits of #4704 .
But I'm currently blocked on this mypy issue: python/mypy#18775