haskell-hoogle: adapt to GHC docs split

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.

pkgs/development/haskell-modules/hoogle.nix

@@ -79,7 +79,7 @@ stdenv.mkDerivation {
     }
 
     echo importing builtin packages
-    for docdir in ${ghc}/${docLibGlob}/*; do
+    for docdir in ${ghc.doc}/${docLibGlob}/*; do
       name="$(basename $docdir)"
       ${opts isGhcjs ''docdir="$docdir/html"''}
       if [[ -d $docdir ]]; then