Come up with a better way of excluding packages that can't build on a particular GHC

This is very common, and we're likely to be in situations where most packages won't build with that version, e.g. when we do https://github.com/input-output-hk/cardano-haskell-packages/issues/173.

So we want some way of doing this that doesn't require manually listing out every version that doesn't work!