Make distinction in apt between removed and purged pkgs

This fixes #4972 by allowing removed packages to be purged using
pkg.purge (and pkg.purged states).
Return empty dict for list_pkgs(removed=True) where not supported

This commit implements the new 'removed' kwarg added to apt (and the
pkg.purged state) in 806342d.
Merge pull request #5503 from terminalmage/issue4972

Allow pkg.purged to purge partially removed packages