flake: add initial flake-parts module

flake/default.nix

@@ -1,6 +1,7 @@
 {
   imports = [
     ./dev
+    ./flake-modules
     ./lib.nix
     ./legacy-packages.nix
     ./overlays.nix

flake/flake-modules/default.nix

@@ -0,0 +1,18 @@
+{ inputs, ... }:
+let
+  # Modules for the flakeModules output and the default module
+  defaultModules = { };
+
+  # Modules for the flakeModules output, but not the default module
+  extraModules = {
+    default.imports = builtins.attrValues defaultModules;
+  };
+in
+{
+  imports = [
+    inputs.flake-parts.flakeModules.flakeModules
+    extraModules.default
+  ];
+
+  flake.flakeModules = defaultModules // extraModules;
+}