Bugfix: new loader feature was missing cacheability

#20675 included a small addition to the basic AMD loader that ships inside ember-source for use in node. I added the ability to register a whole preexisting module.

But when using that feature, we don't stick the results into the seen cache, so subsequent requests will see an empty module.

packages/loader/lib/index.js

@@ -74,7 +74,7 @@ var define, require;
 
     var result = callback.apply(this, reified);
     if (!deps.includes('exports') || result !== undefined) {
-      exports = result;
+      exports = seen[name] = result;
     }
 
     return exports;