Defining a module `foo` results in `foo.foo` being available

Run `maturin new -b pyo3 foo`, followed by `maturin dev` in a venv.

```
>>> import foo
>>> foo.foo
<module 'foo.foo' from '/tmp/venv/lib/python3.9/site-packages/foo/foo.cpython-39-x86_64-linux-gnu.so'>
```

I would expect `foo.foo` to be undefined. The same can be observed when using rust-cpython.

Versions:

* maturin 0.14.8
* Python 3.9.2
* pip 20.3.4