refactor(build): explicitly mention src folder in imports

Export files are now directly under the module folder,
e.g. `core/core.js`. With this, an import like `core/core`
won’t need a path mapping (e.g. via `System.paths`) any more.
This adds the `src` folder to all other import statements as well.

modules/benchmarks_external/src/naive_infinite_scroll/app.dart

@@ -3,7 +3,7 @@ library scroll_app;
 import 'dart:async';
 import 'dart:html';
 import 'package:angular/angular.dart';
-import 'package:e2e_test_lib/benchmark_util.dart';
+import 'package:e2e_test_lib/src/benchmark_util.dart';
 
 @Component(
     selector: 'scroll-app',