Not detect PyVISA

pipreqs does not detect [PyVISA](https://pyvisa.readthedocs.io/en/master/) and write to **requirements.txt** when using

```py
import visa
```

Reproduce the issue with a project containing `import visa`, then inspect how pipreqs maps imports to distributions before generating `requirements.txt`. Done means the generated file includes PyVISA and preserves the other detected dependencies; no specific source file or test is named in the issue.

Written by the indexing model from the issue text.