haskell-generic-builder: add support for overriding pre/post fixup phases
pandoc: $out/nix-support must be erased in the postFixup phase
all-packages.nix: drop trailing whitespace
haskell-modules: make the result of an overridden derivation an overridable derivation again
haskell-generic-builder: add support for building hyperlinked sources with Haddock
haskell-generic-builder: enable Hoogle support in Haddock phase