Fix use mistakes

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     |  no
| Applies to    | all 

On http://symfony.com/doc/current/components/config/resources.html at "new YamlUserLoader", we have also a missing "use" statement. But i'm not sure if we need there one.
If we add a ``use`` statement for the ``PathPackage`` class, we should also add one for the ``AssetsHelper``.

@mbutkereit @WouterJ What do you think?