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.

[Shimuuar]

Current state of support of public sublibraries is quite curious:

1. When package is built by nix all sublibraries are built and installed. There's no way to build package per component
2. Nix can build packages which depends on sublibrary in another package just fine.
3. Yet in environment created by ghcWithPackages modules from public sublibraries couldn't be imported. It gives following error:

    Could not load module ‘Bench.Vector.Algo.Leaffix’
    It is a member of the hidden package ‘vector-0.13.2.0:benchmarks-O2‘

In similar way cabal build cannot find sublibraries in an environment and tries to install them separately