(Even empty) subfolders hide valid imports

A `numpy` subfolder in the project hides the `numpy` pypi package from `requirements.txt`, even if empty and non-functional.

Repro steps
-
```
mkdir temp
cd temp
echo import matplotlib > bug.py
echo import numpy >> bug.py
echo "print(numpy.__version__)" >> bug.py

py bug.py
pipreqs --print

mkdir numpy

py bug.py
pipreqs --print
```

Expected (after the second run)
-
```
matplotlib=...
numpy=...
```

Actual
-
```
matplotlib=...
```

Start by reproducing the issue in a temporary project with bug.py, the empty numpy folder, and the shown pipreqs --print commands. Trace how pipreqs resolves imports when scanning the project; done means the output still includes both matplotlib and numpy after the local empty folder is created.

Written by the indexing model from the issue text.