Preserve lowered extra build dependencies (#15038)

## Summary

I should've noticed this during review -- my bad -- but it looks like
after lowering, we're converting back to `uv_pep508::Requirement`. This
is mostly okay, but it's lossy for some lowerings. For example, we lose
index pinning. With this PR, we now preserve the lowered types
(`Requirement`).

Closes https://github.com/astral-sh/uv/issues/15037.

crates/uv-distribution/src/lib.rs

@@ -3,8 +3,9 @@ pub use download::LocalWheel;
 pub use error::Error;
 pub use index::{BuiltWheelIndex, RegistryWheelIndex};
 pub use metadata::{
-    ArchiveMetadata, BuildRequires, ExtraBuildRequires, FlatRequiresDist, LoweredRequirement,
-    LoweringError, Metadata, MetadataError, RequiresDist, SourcedDependencyGroups,
+    ArchiveMetadata, BuildRequires, FlatRequiresDist, LoweredExtraBuildDependencies,
+    LoweredRequirement, LoweringError, Metadata, MetadataError, RequiresDist,
+    SourcedDependencyGroups,
 };
 pub use reporter::Reporter;
 pub use source::prune;