Support multiple public libraries

[expipiplus1]

Cabal 3 supports multiple public libraries in a single package (and support is hopefully coming to hackage soon (haskell/hackage-server#852)), it would be great if cabal2nix (and hackage2nix) would support such a feature.

I'm not sure exactly what shape the output could take, perhaps putting sub-libraries in passthru would be cool.

[gwils]

This would be helpful for me and my team.