Respect packages in stack.yaml `nix` section

[nc6]

If additional dependencies are specified in stack.yaml, as per:

nix:
  packages: [ openssl ]

they should be added to the package configuration using addBuildDepend for the packages defined in default.nix.

[4e6]

Thanks. Yes, this should be straightforward, I don't see any problems implementing this.
Currently, I am doing some refactoring, preparing to PR my cabal2nix fork. I'll try to look closer on this weekend.

[4e6]

Link to appropriate Stack docs https://docs.haskellstack.org/en/stable/nix_integration/#configuration