Fix stale exports/imports/definitions after unregister_package

unregister_package() called retain_nodes() to remove nodes associated
with a package, but did not clean up the exports, defined, and imports
maps. This left stale NodeIndex references that could cause panics or
incorrect behavior when those maps were subsequently accessed.

Now cleans up all three maps before removing nodes from the graph.
Added a test that verifies exports are properly cleaned up after
unregistering a package.

Author: sumleo

crates/wac-graph/src/graph.rs

@@ -543,6 +543,15 @@ impl CompositionGraph {
             "invalid package id"
         );
 
+        // Remove exports and definitions associated with the package before
+        // removing nodes, as retain_nodes invalidates the node indices.
+        self.exports
+            .retain(|_, n| self.graph[*n].package != Some(package));
+        self.defined
+            .retain(|_, n| self.graph[*n].package != Some(package));
+        self.imports
+            .retain(|_, n| self.graph[*n].package != Some(package));
+
         // Remove all nodes associated with the package
         self.graph
             .retain_nodes(|g, i| g[i].package != Some(package));
@@ -1959,4 +1968,39 @@ mod test {
             UnexportError::MustExportDefinition
         ));
     }
+
+    #[test]
+    fn it_cleans_up_exports_on_unregister_package() {
+        let mut graph = CompositionGraph::new();
+        let bytes = wat::parse_str(
+            r#"(component
+                (import "f" (func))
+                (export "g" (func 0))
+            )"#,
+        )
+        .unwrap();
+
+        let package = wac_types::Package::from_bytes(
+            "test:pkg",
+            None,
+            bytes,
+            graph.types_mut(),
+        )
+        .unwrap();
+        let pkg_id = graph.register_package(package).unwrap();
+
+        // Create an instantiation and export an alias
+        let inst_id = graph.instantiate(pkg_id);
+        let alias_id = graph.alias_instance_export(inst_id, "g").unwrap();
+        graph.export(alias_id, "g").unwrap();
+
+        assert!(graph.get_export("g").is_some());
+
+        // Unregistering the package should clean up exports
+        graph.unregister_package(pkg_id);
+        assert!(
+            graph.get_export("g").is_none(),
+            "exports should be cleaned up after unregister_package"
+        );
+    }
 }