Fix packaging.database.Distribution.list_distinfo_files (#12785).

This method was supposed to return only the file under the dist-info
directory, but it actually returned all installed files.

The tests didn’t catch this because they were flawed; I updated them.
Thanks to Nadeem Vawda and Jeremy Kloth for testing.

As a bonus, the removal of os.path.relpath use should also fix the
Windows buildbots.
2 files changed