Execute haddock concurrently with builds

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 pkgA`dependsOn`build pkgB` we've got:

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