move modules into nixosModules, add autoimport

flake-parts/modules.nix

@@ -1,7 +1,10 @@
 # export some of our flake moduels for re-use in other projects
-{ ...
+{ lib
+, self
+, ...
 }: {
   flake.modules.flake-parts = {
     writers = ./writers;
   };
+  flake.nixosModules = lib.mapAttrs (_: nix: { imports = [ nix ]; }) (self.lib.findNixFiles ../nixosModules);
 }