Removed misleading log message

"No is_installed implementation for '%s'; using slow Python fallback instead!"
Uninstalled packages must have a retrieval method

On Debian without PackageKit, we could get e.g.

Missing retrieval method for package 'python-gobject-2:2.28.6-10:x86_64'

Use a variant type to ensure we always have enough information to tell
the user which native package to install.