Revert "Ensure app/foo/index.js can be looked up with foo:main"

This reverts commit 9fae5ba as it has
reported that it breaks some normal lookup patterns.

I'd still like to bring back this change, but we'll need to investigate
the reported issues and figure out how to mitigate.

addon/resolvers/classic/index.js

@@ -19,7 +19,7 @@ export class ModuleRegistry {
     return Object.keys(this._entries);
   }
   has(moduleName) {
-    return require.has(moduleName);
+    return moduleName in this._entries;
   }
   get(moduleName) {
     return require(moduleName);

tests/unit/resolvers/classic/basic-test.js

@@ -386,17 +386,6 @@ test("store:main is looked up as prefix/store", function(assert) {
   resolver.resolve('store:main');
 });
 
-test("store:main is looked up as prefix/store/index.js", function(assert) {
-  assert.expect(1);
-
-  define('appkit/store/index', [], function(){
-    assert.ok(true, 'store:main was looked up');
-    return 'whatever';
-  });
-
-  resolver.resolve('store:main');
-});
-
 test("store:posts as prefix/stores/post", function(assert) {
   assert.expect(1);