Use local don't work with venvs

When I try to use pipreqs on an active venv, packages are duplicated if they exist both in the system and in the active venv.

Simple example generated requirements:
```
pandas==2.2.3 (from system)
pandas==2.2.1 (from venv)
```

Sometimes packages are found but not written in the requirements...

Linux distro tested:
- Oracle Linux (RHEL 8.10)
- Ubuntu (24.04.2[LTS])

CLI used:
- bash
- zsh

No source file, test, or exact command is identified in the issue. Reproduce the report by running pipreqs from an active venv where a package also exists system-wide; done means the generated requirements contain one correct entry per package and do not omit discovered packages.

Written by the indexing model from the issue text.