Fix regression in apt.py

This fixes a regression introduced in a43a2af a few days ago.
Prevent future parsing issues from crippling apt.list_pkgs

This causes _pkgname_without_arch() to Log an error and return the
passed pkg name if the stripped version ends up being an empty string.
rsplit, not split!
Merge pull request #6101 from terminalmage/issue6090

Fix regression in apt.py