xmonad-with-packages: add xmonad-contrib to packages by default

This is more newbie-friendly; a user without much Nix knowledge would
"nix search" for xmonad, see xmonad-with-packages, and just
immediately install it. This basically never works because most xmonad
users use xmonad-contrib. If we add xmonad-contrib to the packages
list for xmonad by default, then the new user's straightforward action
will just work, most of the time.