pkgs/haskell-modules: expose haddockDir as passthru variable

WIP

If the `hoogle.nix` file wants to have any sane chance of finding haddock
outputs, the packages need to export the haddock folders as an identifier.

A few TODOs still stand, like passing self to the package instead of needing to
pass it to `haddockDir`. Maybe the exported identifier should be integrated into
the fixpoint somehow instead of using `passthru`?