Add `From` for `FlatDistributions` struct (#8800)

Co-authored-by: konsti <konstin@mailbox.org>

crates/uv-resolver/src/flat_index.rs

@@ -226,3 +226,10 @@ impl From<FlatDistributions> for BTreeMap<Version, PrioritizedDist> {
         distributions.0
     }
 }
+
+/// For external users.
+impl From<BTreeMap<Version, PrioritizedDist>> for FlatDistributions {
+    fn from(distributions: BTreeMap<Version, PrioritizedDist>) -> Self {
+        Self(distributions)
+    }
+}