Respect named --index and --default-index values in tool.uv.sources (#7910)

## Summary

If you pass a named index via the CLI, you can now reference it as a
named source. This required some surprisingly large refactors, since we
now need to be able to track whether a given index was provided on the
CLI vs. elsewhere (since, e.g., we don't want users to be able to
reference named indexes defined in global configuration).

Closes https://github.com/astral-sh/uv/issues/7899.

crates/uv-dispatch/src/lib.rs

@@ -156,7 +156,7 @@ impl<'a> BuildContext for BuildDispatch<'a> {
         self.sources
     }
 
-    fn index_locations(&self) -> &IndexLocations {
+    fn locations(&self) -> &IndexLocations {
         self.index_locations
     }
 
@@ -350,6 +350,7 @@ impl<'a> BuildContext for BuildDispatch<'a> {
             self,
             self.source_build_context.clone(),
             version_id,
+            self.index_locations,
             sources,
             self.config_settings.clone(),
             self.build_isolation,