mypy.ini module-specific override doesn't work without `__init__.py`

In the following layout:

- mypy.ini contains
```
[mypy]
disallow_untyped_defs = True

[mypy-a.b.*]
disallow_untyped_defs = False
```
- `a` contains `__init__.py` and a directory `a/b`
- `a/b` contains `c.py` with an untyped def

Mypy will complain about missing type annotations in `a.b.c`. If I create `a/b/__init__.py`, the errors go away. This is a regression in v0.800.