Allow registries to pre-provide core metadata (#15644)

## Summary

This PR adds support for the `application/vnd.pyx.simple.v1` content
type, similar to `application/vnd.pypi.simple.v1` with the exception
that it can also include core metadata for package-versions directly.

crates/uv-resolver/src/version_map.rs

@@ -4,6 +4,7 @@ use std::ops::RangeBounds;
 use std::sync::OnceLock;
 
 use pubgrub::Ranges;
+use rustc_hash::FxHashMap;
 use tracing::instrument;
 
 use uv_client::{FlatIndexEntry, OwnedArchive, SimpleMetadata, VersionFiles};
@@ -17,7 +18,7 @@ use uv_distribution_types::{
 use uv_normalize::PackageName;
 use uv_pep440::Version;
 use uv_platform_tags::{IncompatibleTag, TagCompatibility, Tags};
-use uv_pypi_types::{HashDigest, Yanked};
+use uv_pypi_types::{HashDigest, ResolutionMetadata, Yanked};
 use uv_types::HashStrategy;
 use uv_warnings::warn_user_once;
 
@@ -57,12 +58,25 @@ impl VersionMap {
         let mut stable = false;
         let mut local = false;
         let mut map = BTreeMap::new();
+        let mut core_metadata = FxHashMap::default();
         // Create stubs for each entry in simple metadata. The full conversion
         // from a `VersionFiles` to a PrioritizedDist for each version
         // isn't done until that specific version is requested.
         for (datum_index, datum) in simple_metadata.iter().enumerate() {
+            // Deserialize the version.
             let version = rkyv::deserialize::<Version, rkyv::rancor::Error>(&datum.version)
                 .expect("archived version always deserializes");
+
+            // Deserialize the metadata.
+            let core_metadatum =
+                rkyv::deserialize::<Option<ResolutionMetadata>, rkyv::rancor::Error>(
+                    &datum.metadata,
+                )
+                .expect("archived metadata always deserializes");
+            if let Some(core_metadatum) = core_metadatum {
+                core_metadata.insert(version.clone(), core_metadatum);
+            }
+
             stable |= version.is_stable();
             local |= version.is_local();
             map.insert(
@@ -104,6 +118,7 @@ impl VersionMap {
                 map,
                 stable,
                 local,
+                core_metadata,
                 simple_metadata,
                 no_binary: build_options.no_binary_package(package_name),
                 no_build: build_options.no_build_package(package_name),
@@ -141,6 +156,14 @@ impl VersionMap {
         }
     }
 
+    /// Return the [`ResolutionMetadata`] for the given version, if any.
+    pub fn get_metadata(&self, version: &Version) -> Option<&ResolutionMetadata> {
+        match self.inner {
+            VersionMapInner::Eager(_) => None,
+            VersionMapInner::Lazy(ref lazy) => lazy.core_metadata.get(version),
+        }
+    }
+
     /// Return the [`DistFile`] for the given version, if any.
     pub(crate) fn get(&self, version: &Version) -> Option<&PrioritizedDist> {
         match self.inner {
@@ -352,6 +375,8 @@ struct VersionMapLazy {
     stable: bool,
     /// Whether the version map contains at least one local version.
     local: bool,
+    /// The pre-populated metadata for each version.
+    core_metadata: FxHashMap<Version, ResolutionMetadata>,
     /// The raw simple metadata from which `PrioritizedDist`s should
     /// be constructed.
     simple_metadata: OwnedArchive<SimpleMetadata>,