Using methods defined in __init__.py within the module

Use relative imports:

from . import win, lose

Yes, just import the __init__.py module (via either an absolute or relative import, it doesn't really matter).

I never like relative imports, so I'd do so with import mypackage in mypackage.foo, which imports the __init__.py just like a relative import does, and then using it there. I also don't like putting anything in __init__.py though generally, so perhaps you should consider the shared common file anyhow.