Avoid using a nested WeakMap for manager instances for a given owner.

The first level WeakMap is sufficient to prevent memory leaks. The only
potential value that the second level WeakMap provides is to allow us to
_unload_ code during the lifetime of a single owner instance, which is
really not something that we support.

Having nested WeakMap's breaks some tooling around memory leak
investigation that we commonly use.

packages/@glimmer/runtime/lib/managers/index.ts

@@ -42,7 +42,7 @@ const HELPER_MANAGERS = new WeakMap<
 
 const OWNER_MANAGER_INSTANCES: WeakMap<
   Owner,
-  WeakMap<ManagerFactory<Owner>, unknown>
+  Map<ManagerFactory<Owner>, unknown>
 > = new WeakMap();
 const UNDEFINED_MANAGER_INSTANCES: WeakMap<ManagerFactory<Owner>, unknown> = new WeakMap();
 
@@ -107,7 +107,7 @@ function getManagerInstanceForOwner<D extends ManagerDelegate>(
     managers = OWNER_MANAGER_INSTANCES.get(owner);
 
     if (managers === undefined) {
-      managers = new WeakMap();
+      managers = new Map();
       OWNER_MANAGER_INSTANCES.set(owner, managers);
     }
   }