get_class fails with top level packages.

Hello.
In an attemp to extend the shipping "app" we came into a problem.

We copied the shipping package into our project's package. So we have 
MyProject/
  shipping/
     repository.py
  MyProject/
     settings.py
....

But after several tries and debugging, the get_class (and more precisely, get_classes) in oscar/core/loading.py was not picking the classes in this package and son, it defaulted to the oscar's system wide shipping module.

After a grep in oscar, it seems that everytime that Repository is required, it is loaded with get_classs. So it should not need any additional bonding other than overriding in INSTALLED_APPS:

INSTALLED_APPS = INSTALLED_APPS +  get_core_apps(['shipping'])

It turns out that when get_classes generating the module path, if it is a top level package, it will generate something like : shipping.shipping.repository instead of shipping.repository.

The problem is that _get_app_module_path assumes there's always a dot in the module's name. get_classes should check that there is a dot in the module label before calling _get_app_module_path.