mypy cache folder causes generated requirements to be empty

Related to #73.

### Steps to reproduce
- Create **foo.py**:
```python
import requests
```
- Run the following commands:
```
$ pipreqs --print
requests==2.28.1
INFO: Successfully output requirements

$ mypy foo.py
Success: no issues found in 1 source file

$ pipreqs --print

INFO: Successfully output requirements
```
I would expect 'requests' to still be in the generated requirements after running mypy.
### Description
Since mypy creates a subfolder called `.mypy_cache/3.10/requests`, pipreqs will add it to 'candidates' which causes the import to be satisfied, even though foo.py is obviously not going to attempt to import that folder. Ignoring `.mypy_cache` is the simple fix, but maybe folders shouldn't be considered packages unless they contain an `__init__.py`?
https://github.com/bndr/pipreqs/blob/a593d27e3d9fcdecc0fbf385ef43116cccad71ec/pipreqs/pipreqs.py#L104-L108
https://github.com/bndr/pipreqs/blob/a593d27e3d9fcdecc0fbf385ef43116cccad71ec/pipreqs/pipreqs.py#L143-L144

Start in pipreqs/pipreqs.py at the linked lines around candidate discovery and package handling, then reproduce the commands from the issue with and without .mypy_cache. Done means running pipreqs after mypy still reports requests in the generated requirements rather than treating the cache directory as the import source.

Written by the indexing model from the issue text.