Allow building mixed projects with __init__.pyi file

[vinhowe]

This makes it possible to hand author a pyi stub file for the root module until we have something like PyO3/pyo3#510.

[vinhowe]

This doesn't seem to work like I hoped. I'll have to figure out what's actually going on.

[vinhowe]

To anyone trying to do this, it looks like you need an __init__.py file after all. The .so file that Maturin puts in the directory next to it is just treated by Python as a submodule, so none of its members are available from the root module, obviously, unless you import them.

So __init__.py just looks like this:

from .package_name import *