fix: expose types to implement custom ResolverProvider (#1862)

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## Summary

To integrate `uv` into `pixi` I need to specify a custom
`ResolverProvider` to be able to specify that some packages are already
installed by conda and should not be touched. However, some of the types
required to implement your own `ResolverProvider` were not accessible
through the public API. This PR basically adds them.

## Test Plan

I didnt add an explicit test for this.

crates/uv-resolver/src/version_map.rs

@@ -181,6 +181,14 @@ impl From<FlatDistributions> for VersionMap {
     }
 }
 
+impl From<BTreeMap<Version, PrioritizedDist>> for VersionMap {
+    fn from(value: BTreeMap<Version, PrioritizedDist>) -> Self {
+        Self {
+            inner: VersionMapInner::Eager(value),
+        }
+    }
+}
+
 /// A lazily initialized distribution.
 ///
 /// This permits access to a handle that can be turned into a resolvable