don't delete contets of lib dir, but replace requirements instead

As there are some libs that can't be installed via *pip*, it's a common case to install them manually (i.e. copying them to the lib directory).

This change takes this into account this, and avoids unlinking files that are not going to be replaced.

The drawback is that dependencies no longer listed in the requirements file are not deleted anymore, but this mimics **pip install -r**'s default behavior more closely (i.e. users manually uninstall no-longer required packages)