Add option for defining flake-parts modules for downstream flakes.

modules/flakeModules.nix

@@ -0,0 +1,45 @@
+{ config, self, lib, flake-parts-lib, ... }:
+let
+  inherit (lib)
+    filterAttrs
+    mapAttrs
+    mkOption
+    optionalAttrs
+    types
+    ;
+  inherit (flake-parts-lib)
+    mkAliasOptionModule
+    ;
+
+  flakeModulesOption = mkOption {
+    type = types.lazyAttrsOf types.deferredModule;
+    default = { };
+    apply = mapAttrs (k: v: {
+      _file = "${toString self.outPath}/flake.nix#flakeModules.${k}";
+      imports = [ v ];
+    });
+    description = ''
+      flake-parts modules for use by other flakes.
+
+      If the flake defines only one module, it should be flakeModules.default.
+
+      You can not use this option in defining the flake's own `imports`. Instead, you can
+      put the module in question into its own file and reference it both in `imports` and
+      export it with this option.
+    '';
+  };
+in
+{
+  options = {
+    flake = mkOption {
+      type = types.submoduleWith {
+        modules = [
+          (mkAliasOptionModule [ "flakeModule" ] [ "flakeModules" "default" ])
+          {
+            options.flakeModules = flakeModulesOption;
+          }
+        ];
+      };
+    };
+  };
+}