Implement ops::Index with PackageId for Resolve graph

While iterating through dependency packages, I find myself needing to
look up dependency nodes recursively through the graph, in the same
way that I need `Package` information for its `PackageId` via the
already-existing `ops::Index` implementation on `Metadata`.  This is
simpler if the indexing operation is also available on a `Resolve`d
graph.

src/lib.rs

@@ -216,7 +216,7 @@ impl Metadata {
 impl<'a> std::ops::Index<&'a PackageId> for Metadata {
     type Output = Package;
 
-    fn index(&self, idx: &'a PackageId) -> &Package {
+    fn index(&self, idx: &'a PackageId) -> &Self::Output {
         self.packages
             .iter()
             .find(|p| p.id == *idx)
@@ -272,6 +272,17 @@ pub struct Resolve {
     pub root: Option<PackageId>,
 }
 
+impl<'a> std::ops::Index<&'a PackageId> for Resolve {
+    type Output = Node;
+
+    fn index(&self, idx: &'a PackageId) -> &Self::Output {
+        self.nodes
+            .iter()
+            .find(|p| p.id == *idx)
+            .unwrap_or_else(|| panic!("no Node with this id: {:?}", idx))
+    }
+}
+
 #[derive(Clone, Serialize, Deserialize, Debug)]
 #[cfg_attr(feature = "builder", derive(Builder))]
 #[non_exhaustive]