Add in-URL credentials to store prior to creating requests (#2446)

## 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 https://github.com/astral-sh/uv/issues/2444.

## Test Plan

`cargo run pip install` against an authenticated AWS registry.

crates/pypi-types/src/base_url.rs

@@ -57,21 +57,6 @@ pub struct BaseUrl(
 );
 
 impl BaseUrl {
-    /// Parse the given URL. If it's relative, join it to the current [`BaseUrl`]. Allows for
-    /// parsing URLs that may be absolute or relative, with a known base URL.
-    pub fn join_relative(&self, url: &str) -> Result<Url, url::ParseError> {
-        match Url::parse(url) {
-            Ok(url) => Ok(url),
-            Err(err) => {
-                if err == url::ParseError::RelativeUrlWithoutBase {
-                    self.0.join(url)
-                } else {
-                    Err(err)
-                }
-            }
-        }
-    }
-
     /// Return the underlying [`Url`].
     pub fn as_url(&self) -> &Url {
         &self.0