Error on import_name when a backend is a root package

Hi!                                                                             
When trying to make a custom backend for this application, I run into a problem when it tries to load the backend module.
I was following the instructions to create a custom backend explained on the documentation, using a django app and creating the `PaymentProccesor` on the `__init__.py` file.
Dbuggging the point of failure, I found that since the module of my backend was a root module (without parent modules), the `import_name` method was failing to import it. For some reason, it was using the `getpaid` module as the parent module (probably because that's the parent module of `utils`?), and thus failing to import my backend module.
                                                                                
I made a little change on the `import_name` function that seems to have fixed this issue for me, and works correctly with the `dummy` backend, so I guess it didn't break anything.
Please consider adding this modification to your repo.                          
                                                                                
Cheers!