Disallow deprecated language features

[piegamesde]

I'd like to keep NixOS/nixpkgs#365186 from regressing, so would it be possible to introduce rules that forbid these kind of strings? More broadly speaking, there are a couple of other highly deprecated features within the Nix language that nobody is using, and it might be a good idea to check for that as well.

[philiptaron]

Yes, I'd welcome that.