Fixes Embroider Usage & ensures tests no longer mutate the global state

 
Embroider wants to take over global defines, in-order to give them some specific treatments. Our tests in ember-resolver, where mutating the global loader by declaring it’s own global defines. 

Polluting the global state isn’t best practice, especially if there is a public API abstracting the global state. This PR 1) ensures the classic resolver always uses that abstraction 2) provides a test specific piece of state which implements the abstraction.

addon/resolvers/classic/index.js

@@ -21,8 +21,8 @@ export class ModuleRegistry {
   has(moduleName) {
     return moduleName in this._entries;
   }
-  get(moduleName) {
-    return require(moduleName);
+  get(...args) {
+    return require(...args);
   }
 }
 
@@ -471,7 +471,7 @@ const Resolver = EmberObject.extend({
   },
 
   _extractDefaultExport(normalizedModuleName) {
-    let module = require(normalizedModuleName, null, null, true /* force sync */);
+    let module = this._moduleRegistry.get(normalizedModuleName, null, null, true /* force sync */);
 
     if (module && module['default']) {
       module = module['default'];