export module as system-independent flake output

Flake outputs are a mixture of system-dependent and system-independent
sets, and flake-utils doesn't do much to distinguish one from the other.
Because of that, the `age` NixOS module currently has to be acessed as
`agenix.nixosModules.${system}.age`, rather than the documented
`agenix.nixosModules.age`.

To remedy that, (conceptually) split `outputs` in two, let flake-utils
handle the system-dependent half, and merge them to form the actual
outputs.  The names for the two halves were taken from [1].

[1]: NixOS/nix#3843 (comment)

flake.nix

@@ -4,10 +4,14 @@
   inputs.flake-utils.url = "github:numtide/flake-utils";
 
   outputs = { self, nixpkgs, flake-utils }:
-    flake-utils.lib.eachDefaultSystem (system:
-      {
+    let
+      exports = {
         nixosModules.age = import ./modules/age.nix;
-        packages = nixpkgs.legacyPackages.${system}.callPackage ./default.nix {};
+      };
+      outputs = flake-utils.lib.eachDefaultSystem (system: {
+        packages = nixpkgs.legacyPackages.${system}.callPackage ./default.nix { };
         defaultPackage = self.packages.${system}.agenix;
       });
+    in
+    exports // outputs;
 }