Check for overwrite conflict sooner

Minor thing, but I noticed this on a large project dir that it checks for the dependencies before warning you that it won't save or print anything because the target requirements.txt already existed.

Could tell the user sooner instead of making them wait twice ;)

fwiw, I also just noticed that this took longer because I had a virtualenv in the project directory, I wonder if it is worth it to warn the user about that (especially if it's the active env)?

Cheers!

Start at the pipreqs CLI flow that discovers dependencies and handles an existing target requirements.txt. Confirm the overwrite conflict is detected before dependency scanning so the user is not kept waiting; the virtualenv warning is an additional question left for project guidance.

Written by the indexing model from the issue text.