stack-to-nix: generate more nix scaffolding for projects

[rvl]

After running stack-to-nix, there is still a fair amount of code to add (copy&paste) before you have a working build.

[rvl]

Implemented by input-output-hk/nix-tools#29.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.