allow system definitions to be done in folders

systems/default.nix

@@ -6,7 +6,11 @@ let
   # call like `mkSystem "x86_64-linux" "<hostname>.eh22.intern"`
   mkSystem =
     systemType: name:
-    nixpkgs.lib.nixosSystem {
+    let
+      lib = nixpkgs.lib;
+      systemModule = if lib.pathIsDirectory ./${name} then ./${name}/system.nix else ./${name}.nix;
+    in
+    lib.nixosSystem {
       system = systemType;
       specialArgs = flake.inputs;
       modules = [
@@ -17,8 +21,7 @@ let
 
         ../modules/base_system.nix
         ../modules/user_account.nix
-        #../modules/mail_relay.nix
-        ./${name}.nix
+        systemModule
 
         (
           let