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

• Changes have tests
• News fragment added in newsfragments/.
  (See documentation for details)