Referring to own packages in nixosModules import

[waffle8946]

@glab

      # Make hello runnable with `nix run`
      apps = {
        default = {
          type = "app";
          program = "${self.packages.${system}.default}/bin/geotopo";
        };
      };

You don’t need this, just set meta.mainProgram = "geotopo"; in your package.
Also, using venvs is definitely a strange way to package, just use buildPythonApplication or such: https://nixos.org/manual/nixpkgs/unstable/#building-packages-and-applications

@glab

web-service-env = pkgs.web-service-env;

Pass inputs into the module system and use inputs.self.packages.${pkgs.stdenv.hostPlatform.default}.default