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/directives/directives.js

@@ -0,0 +1,4 @@
+export * from './src/ng_if';
+export * from './src/ng_non_bindable';
+export * from './src/ng_repeat';
+export * from './src/ng_switch';