Support GHC toolchains

[nmattia]

This PR in rules_haskell changed the way Haskell toolchains are handled: tweag/rules_haskell#610

The toolchains are not referenced by name anymore; rather the best toolchain is picked. As an example, the rules_haskell WORKSPACE does not give the bindist toolchains a name anymore: https://github.com/tweag/rules_haskell/blob/12af32dee83ba654cc01a84a4b0f105e80c243a7/WORKSPACE#L102-L105

This means we can't generally pass the ghc_workspace to hazel anymore. Instead Hazel should, when needed, figure out the toolchain that's being used.