(PUP-1085) Make Pacman provider group-aware

The Archlinux Pacman package manager has a concept of package groups,
which due to the provider not checking for a group being installed
caused all contained packages to be reinstalled.

This commit uses pacman -Qg in addition to pacman -Q to check for
installed groups.

Because groups don't have versions this will still cause all packages in
a group to be reinstalled if ensure is set to latest.