feat: simplify flake-parts imports

flake/formatter.nix

@@ -0,0 +1,43 @@
+{ inputs, ... }:
+{
+  imports = [
+    inputs.treefmt-nix.flakeModule
+  ];
+
+  perSystem = { pkgs, ... }: {
+    # formatter is set by treefmt to:
+    # formatter = lib.mkIf config.treefmt.flakeFormatter (lib.mkDefault config.treefmt.build.wrapper);
+    treefmt = {
+      projectRootFile = "flake.nix";
+      programs = {
+        nixfmt = {
+          enable = true;
+          package = pkgs.nixpkgs-fmt;
+        };
+        deadnix.enable = true;
+        statix.enable = true;
+        shfmt = {
+          enable = true;
+          indent_size = 4;
+          simplify = true;
+          # needed to replicate what my Emacs shfmt does
+          # there is no builtin option for space-redirects
+          package = pkgs.symlinkJoin {
+            name = "shfmt";
+            buildInputs = [ pkgs.makeWrapper ];
+            paths = [ pkgs.shfmt ];
+            postBuild = ''
+              wrapProgram $out/bin/shfmt \
+              --add-flags '-sr'
+            '';
+          };
+        };
+        shellcheck.enable = true;
+      };
+      settings.formatter.shellcheck.options = [
+        "--shell"
+        "bash"
+      ];
+    };
+  };
+}