How to package a Python C Extension such that it is a submodule of a normal Python module? [duplicate]

I wrote a python library with two parts:

1. A Python C extension
2. A Python wrapper for the Python C extension

I would like to be able to package it in such a way that the Python wrapper is the top level module foo and the Python C extension is a submodule located at foo._foo. However I have so far only been able to create two top level modules, foo and _foo.

What do I need to do in setup.py and in the init_foo C function in order to accomplish this?

(My question is subtlety different from this)

Current directory structure:

foo/
    foo/
        __init__.py
    foo.c
    setup.py
    tests.py

setup.py looks something like:

from distutils.core import setup, Extension

module = Extension('_foo',
                   sources=['foo.c'])

setup(name='foo', packages=['foo'], ext_modules=[module])

foo.c looks something like:

PyMODINIT_FUNC init_foo(void) {
    PyObject *m;
    m = Py_InitModule("_foo", FooMethods);
    // ..
}
int main(int argc, char *argv[]) {
    Py_SetProgramName(argv[0])
    Py_Initialize();
    init_pychbase();
}

foo/__init__.py looks something like:

from _foo import _Foo, _Bar, _Baz

class Foo(object):
    def __init__(self):
        self._foo = _Foo()