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]

This would previously raise if self.distribution.data_files was None

[Avasam]

This was typed as object because it's unused. However, distutils types it as int.

So either distutils should change it to object (or Any) to indicate that anything can be passed to level and that subclasses are not intended to implement it.
Or, if some subclasses are meant to implement it, then this should indeed be level: int even if unused.