Refactored `_get_index_url()` to get integration tests for the subprocess backend working.
Keyring support via the 'subprocess' provider can only retrieve a password, not a username-password combo. The username therefor MUST come from the URL.
If the URL obtained from the index does not contain a username then the username from a matching index is used. `_get_index_url()` does that matching.
The problem this refactoring solves is that the URL where a wheel or sdist can be downloaded from does not always start with the index url. Azure DevOps Artifacts Feeds are an example since it replaces the friendly name of the Feed with the GUID of the Feed. Causing `url.startswith(prefix)` to evaluate as `False`.
The new behaviour is to return the index which matches the netloc and has the longest common prefix of the `path` property of the value returned by `urllib.parse.urlsplit()`. The behaviour for resolving ties is unspecified.
