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/angular/angular.js

@@ -0,0 +1,6 @@
+/**
+ * Define public API for Angular here.
+ */
+export * from 'change_detection/change_detection';
+export * from 'core/core';
+export * from 'directives/directives';