Add in-URL credentials to store prior to creating requests

[charliermarsh]

Summary

The authentication middleware extracts in-URL credentials from URLs that pass through it; however, by the time a request reaches the store, the credentials will have already been removed, and relocated to the header. So we were never propagating in-URL credentials.

This PR adds an explicit pass wherein we pass in-URL credentials to the store prior to doing any work.

Closes #2444.

Test Plan

cargo run pip install against an authenticated AWS registry.

[zanieb]

Thanks! I'll investigate other approaches.