BUG: not able to detect local packages

I have pandas installed
```
> pip list | grep pandas
pandas 2.2.0
```
but pipreqs is not able to detect it and tries to resolve it via PyPI. Why?

```sh
> pipreqs . --print

INFO: Not scanning for jupyter notebooks.
WARNING: Import named "pandas" not found locally. Trying to resolve it at the PyPI server.
WARNING: Import named "pandas" was resolved to "pandas:2.2.2" package (https://pypi.org/project/pandas/).
Please, verify manually the final list of requirements.txt to avoid possible dependency confusions.
matplotlib==3.8.2
numpy==1.24.1
numpy==1.21.5
pandas==2.2.2
INFO: Successfully output requirements
```

Start by reproducing the reported `pipreqs . --print` command with locally installed pandas, then trace how pipreqs detects imported packages and falls back to PyPI. Done means a locally installed pandas package is detected without the PyPI warning and the generated requirement reflects the local installation.

Written by the indexing model from the issue text.