Modernize package_index VCS handling

[jaraco]

• Remove pop_prefix parameter, unused.
• Add a test capturing the basic expectation.
• Update _vcs_split_rev_from_url to use modern constructs.
• package-index: Extract fall-through methods _download_vcs and _download_other.
• Extract _resolve_vcs for resolving a VCS from a URL.
• Consolidated all _download_vcs methods into one.
• Replace os.system calls with subprocess calls.
• Prefer tmp_path fixture.

Summary of changes

Closes

Pull Request Checklist

• [x] Changes have tests
• [x] News fragment added in newsfragments/. (See [documentation][PR docs] for details)

[PR docs]: https://setuptools.pypa.io/en/latest/development/developer-guide.html#making-a-pull-request