stack-to-nix: extra-deps repeated in .stack-pkgs.nix #1690

If the same dependency is listed extra-deps of stack.yaml and it also appears in the packages of the resolver, then it will be output twice in .stack-pkgs.nix.

The error will look like:

error: attribute 'Chart' at /home/rodney/iohk/cardano-wallet/nix/.stack-pkgs.nix:8:9 already defined at /home/rodney/iohk/cardano-wallet/nix/.stack-pkgs.nix:6:9