treefmt: flatten the config

By importing the config on the treefmt module level, it makes the config
more flat, and also compatible with non-flake-parts users.

flake.nix

@@ -60,9 +60,10 @@
           inputs.treefmt-nix.flakeModule
           ./effect.nix
           ./shell.nix
-          ./treefmt.nix
         ];
 
+        perSystem.treefmt.imports = [ ./treefmt.nix ];
+
         flake.nixosConfigurations =
           let
             inherit (inputs.nixpkgs.lib) nixosSystem;

treefmt.nix

@@ -1,6 +1,4 @@
-{
-  perSystem = { pkgs, ... }: {
-    treefmt = {
+{ pkgs, ... }: {
   # Used to find the project root
   projectRootFile = "flake.lock";
 
@@ -49,6 +47,4 @@
       includes = [ "*.py" ];
     };
   };
-    };
-  };
 }