Use package name when __main__.py is used

I'm investigating how to package a single file application with fire at the moment.

It seems that in order to have pip install the requirements, a `setup.py` files needs to exist. And `setup.py` requires that a *package* be defined. When this is done, a file, `__main__.py` is created in the package directory and invoked using, `python -m mypackage COMMAND`.

In this case the logic is in `mypackage/__main__.py`, which, when used with fire shows `__main__.py` in the usage output, where `mypackage` display is desirable.