Building hoogle environments with an empty list of packages threw a built-time
error. This was particularly confusing when giving e.g. `(hps: [hps.Cabal])`
which, since `Cabal` is set to `null` (because it’s a distribution package),
would have lead to the same error as an empty list.
It was not useful to throw an error at all, because sometimes one only wants to
build an environment for the distribution packages; also the default value for
packages wasn’t even a valid value, so it is removed.
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.
