Generate haddock for multiple (sub)libraries at once

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](https://github.com/haskell/hackage-server/pull/1279) is deployed, hackage will list the modules of multiple libraries, so a lack of docs will become quite visible.