Avoid panic for invalid, non-base index URLs (#4527)

## Summary

See: https://github.com/astral-sh/uv/issues/4510

crates/uv-client/src/error.rs

@@ -137,6 +137,9 @@ pub enum ErrorKind {
     #[error("Expected a file URL, but received: {0}")]
     NonFileUrl(Url),
 
+    #[error("Expected an index URL, but received non-base URL: {0}")]
+    CannotBeABase(Url),
+
     #[error(transparent)]
     DistInfo(#[from] install_wheel_rs::Error),
 

crates/uv-client/src/registry_client.rs

@@ -270,7 +270,7 @@ impl RegistryClient {
         // Format the URL for PyPI.
         let mut url: Url = index.clone().into();
         url.path_segments_mut()
-            .unwrap()
+            .map_err(|()| ErrorKind::CannotBeABase(index.clone().into()))?
             .pop_if_empty()
             .push(package_name.as_ref())
             // The URL *must* end in a trailing slash for proper relative path behavior