Use index URL instead of package URL for keyring credential lookups (#12651)

Some registries (like Azure Artifact) can require you to authenticate
separately for every package URL if you do not authenticate for the
/simple endpoint. These changes make the auth middleware aware of index
URL endpoints and attempts to fetch keyring credentials for such an
index URL when making a request to any URL it's a prefix of.

The current uv behavior is to cache credentials either at the request
URL or realm level. But with these changes, we also need to cache
credentials at the index level. Note that when uv does not detect an
index URL for a request URL, it will continue to apply the old behavior.

Addresses part of #4056
Closes #4583
Closes #11236
Closes #11391
Closes #11507

docs/configuration/authentication.md

@@ -38,9 +38,9 @@ Authentication can come from the following sources, in order of precedence:
 - A [`.netrc`](https://everything.curl.dev/usingcurl/netrc) configuration file
 - A [keyring](https://github.com/jaraco/keyring) provider (requires opt-in)
 
-If authentication is found for a single net location (scheme, host, and port), it will be cached for
-the duration of the command and used for other queries to that net location. Authentication is not
-cached across invocations of uv.
+If authentication is found for a single index URL or net location (scheme, host, and port), it will
+be cached for the duration of the command and used for other queries to that index or net location.
+Authentication is not cached across invocations of uv.
 
 `.netrc` authentication is enabled by default, and will respect the `NETRC` environment variable if
 defined, falling back to `~/.netrc` if not.