feat: use treefmt-nix and split up flake.nix

flake-modules/package.nix

@@ -0,0 +1,24 @@
+{
+  perSystem =
+    { pkgs, ... }:
+    {
+      packages =
+        let
+          webnsupdate = pkgs.callPackage ../default.nix { };
+        in
+        {
+          inherit webnsupdate;
+          default = webnsupdate;
+          cargo-update = pkgs.writeShellApplication {
+            name = "cargo-update-lockfile";
+            runtimeInputs = with pkgs; [
+              cargo
+              gnused
+            ];
+            text = ''
+              CARGO_TERM_COLOR=never cargo update 2>&1 | sed '/crates.io index/d' | tee -a cargo_update.log
+            '';
+          };
+        };
+    };
+}