Add support for GHCJS

This provides a (AFAICT) fully working GHCJS package set.

@peti Can I get your feedback on how to best merge this in? I'm sure there are some things to consider, like if/how we should specify that a given package supports GHCJS (or more generically, any particular compiler).
Oh, I missed the fact that this was specific to haddock (which I think has the same issue I described).  This refactor looks OK to me, and it might trigger fewer rebuilds overall.