Support all services packages in flake

This allows ex: nix build .#gogol-gmail

note that this causes:

❯ nix flake show 
git+file:///home/imalison/Projects/gogol?ref=refs/heads/addRealFlake&rev=09c59df08df2f9de321532dc011b58ce8785dd62
├───defaultPackage
│   ├───aarch64-darwin omitted (use '--all-systems' to show)
│   ├───aarch64-linux omitted (use '--all-systems' to show)
│   ├───i686-linux omitted (use '--all-systems' to show)
│   ├───x86_64-darwin omitted (use '--all-systems' to show)
error:
       … while evaluating the attribute 'defaultPackage'

         at /nix/store/gf87d4imyf2wdmwbbbpnlz65za5xmvnj-source/flake.nix:51:5:

           50|     packages = builtins.listToAttrs (map mkPackage allPackages);
           51|     defaultPackage = self.packages.${system}.gogol;
             |     ^
           52|   }) // { inherit hoverlay overlay overlays; } ;

       … while evaluating the attribute 'packages.x86_64-linux.gogol'

         at /nix/store/gf87d4imyf2wdmwbbbpnlz65za5xmvnj-source/flake.nix:39:7:

           38|       name = name;
           39|       value = hpkg.${name};
             |       ^
           40|     };

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: cannot build '/nix/store/gyl45vl41zwyknsfjparbfxlbknqm5hb-cabal2nix-gogol.drv' during evaluation because the option 'allow-import-from-derivation' is disabled

The full output when actually running this is:

https://gist.github.com/IvanMalison/5fa038c506aa66f7507686c56f041d83

brendanhay / gogol

Support all services packages in flake #189