Ensure authentication is passed from the index url to distribution files (#1886)

Closes https://github.com/astral-sh/uv/issues/1709
Closes https://github.com/astral-sh/uv/issues/1371

Tested with the reproduction provided in #1709 which gets past the HTTP
401.

Reuses the same copying logic we introduced in
https://github.com/astral-sh/uv/pull/1874 to ensure authentication is
attached to file URLs with a realm that matches that of the index. I had
to move the authentication logic into a new crate so it could be used in
`distribution-types`.

We will want to something more robust in the future, like track all
realms with authentication in a central store and perform lookups there.
That's what `pip` does and it allows consolidation of logic like netrc
lookups. That refactor feels significant though, and I'd like to get
this fixed ASAP so this is a minimal fix.

crates/pypi-types/src/base_url.rs

@@ -76,6 +76,12 @@ impl BaseUrl {
     pub fn as_url(&self) -> &Url {
         &self.0
     }
+
+    /// Convert to the underlying [`Url`].
+    #[must_use]
+    pub fn into_url(self) -> Url {
+        self.0
+    }
 }
 
 impl From<Url> for BaseUrl {