add HaskellNet

Hi, first contribution here. I generated `pkgs/development/libraries/haskell/HaskellNet/default.nix` using `cabal2nix cabal://HaskellNet` and added a line to `pkgs/top-level/haskell-packages.nix`.

Testing using the method here also seems fine
https://nixos.org/wiki/Haskell#Maintaining_your_own_set_of_additional_Haskell_packages

 Did I miss anything?