Last released xmonad doesn't build with GHC 9.0 and haskell-ci doesn't
let us tweak the test matrix easily. Let's hope the stack.yml workflow
is sufficient to test backward compat.
Related: 2a1a18023a
Not everyone has those repositories checked out at those paths. IMHO, it would
be much better to configure these as "optional-packages". In any case, users
should probably configure those in their cabal.project.local.