haskell_haddock compatibility with prebuilt_dependencies

[Fuuzetsu]

We may want to support prebuilt_dependencies of packages in haskell_haddock. This involves:

• finding out paths of prebuilt_dependencies
• adding them to --read-interface

We may be able to get these paths from ghc-pkg. To feed these paths into haddock itself we may want to use run_shell and interpolate or generate a response file with all the args prepopulated (but still needs run_shell most likely). These things can be a bit more granular by abusing files: for every prebuilt dependency, run ghc-pkg and produce a file with the path needed. Then when invoking haddock use $(cat path_of_prebuilt_dep_FOO) $(cat path_of_prebuilt_dep_BAR). This allows us to only invoke ghc-pkg once per every prebuilt dependency across all rules rather than call ghc-pkg for every single prebuilt dep for every single dep in the package which would likely be rather slow.

Before anything, think about whether we actually want this as a feature. prebuilt_dependencies is somewhat of a hack to begin with and maybe we should be encouraging to package more things and use prebuilt_dependencies less rather than encouraging its use.

Created from #47.