fix: use per-extension subdirectory in build_temp to prevent parallel build race (#5196)

[bibekmhj]

Summary

Fixes #5196

Root cause

All extensions share the same build_temp directory when compiled
in parallel with -j. The compiler.compile() call uses
output_dir=self.build_temp for every extension, so .o files
with the same name overwrite each other.

Fix

Derive a per-extension output_dir from ext.name:

ext_build_temp = os.path.join(self.build_temp, *ext.name.split('.'))

pkg.ext1 → build/temp.../pkg/ext1/
pkg.ext2 → build/temp.../pkg/ext2/

Testing

Added unit test verifying the two extensions receive different
output_dir values.

[Avasam]

Please note that https://setuptools.pypa.io/en/latest/development/developer-guide.html#code-conventions-and-other-practices

Finally, the setuptools/_distutils directory should not be modified directly when contributing to the setuptools project. Instead, this directory is maintained as a separated project in https://github.com/pypa/distutils, and periodically merged into setuptools.

and #5196 (comment)

One thing that requires attention though is that changes in the setuptools/_distutils directory need to be addressed in the pypa/distutils repository.