Generate haddock for multiple (sub)libraries at once

[414owen]

Currently, running cabal haddock --haddock-for-hackage seems to produce a tarball containing the docs for one library.

If you pass it the name of a sublibrary, it will generate the docs for that sublibrary only.

Hackage seems to overwrite the docs when you upload a new tarball, so it seems people would need to either fiddle with tarballs manually, or have only one library documented.

Note that when this change is deployed, hackage will list the modules of multiple libraries, so a lack of docs will become quite visible.