Execute haddock concurrently with builds

[mgsloan]

Currently, the haddock runs as part of the build step, at the end of the build step. This means that for subsequent build steps we can be waiting on haddock, even though the subsequent build steps would work just fine (up until its haddock step).

So with haddock on, instead of having action dependencies like build pkgAdependsOnbuild pkgB we've got:

build pkgA `dependsOn` build pkgB
haddock pkgA `dependsOn` build pkgA
haddock pkgB `dependsOn` build pkgB
haddock pkgB `dependsOn` haddock pkgA

[borsboom]

I have plans to overhaul the haddock generation so that it no longer uses Cabal to do the haddocks and that should make this sort of change easier as well.