Allow non-file:// paths to serve as --index-url values

[charliermarsh]

Summary

pip allows these with the following logic:

if os.path.exists(location):  # Is a local path.
    url = path_to_url(location)
    path = location
elif location.startswith("file:"):  # A file: URL.
    url = location
    path = url_to_path(location)
elif is_url(location):
    url = location

Closes #4510.

Test Plan

cargo run pip install --index-url ../packse/index/simple-html/ example-a-961b4c22 --reinstall --no-cache --no-deps