The docs were split at #16167 and broke google support. @peti fixed
build, however hoogle still didn't see packages shipped with GHC. This
patch fixes location of the libraries shipped with GHC.
ghcWithHoogle builds with this patch applied, but it's probably still broken in
the sense that links might point nowhere or that the generated databases and/or
documentation might be incomplete.
* Build most of the stuff on /tmp, not in /nix/store.
* Generate hoogle database for all the dependencies.
* Generate haddock index and contents files.
* Cleanup.
