python.buildenv: don't filter non-python packages

python.buildenv is used to build an env that provides binaries that can
import all modules that were passed in to the env.

Before this change it filtered the propagatedBuildInputs to remove all
non-Python packages, thereby possibly reducing the amount of packages
that were referenced. However, Python packages often don't have non-
Python packages as propagatedBuildInputs. And occasionally, we do want
to be able to add other packages to the env.