build(nix): combine package declaration & expression

default.nix

@@ -1,11 +1,41 @@
 {
-  pkgs ? import <nixpkgs> {
-    inherit system;
-    config = { };
-    overlays = [ ];
-  },
+  pkgs ?
+    import <nixpkgs> {
+      inherit system;
+      config = {};
+      overlays = [];
+    },
   system ? builtins.currentSystem,
-}:
-{
-  catwalk = pkgs.callPackage ./nix/package.nix { };
+}: {
+  catwalk = pkgs.callPackage (
+    {
+      lib,
+      rustPlatform,
+    }:
+      rustPlatform.buildRustPackage {
+        pname = "catppuccin-catwalk";
+        inherit ((lib.importTOML ./Cargo.toml).package) version;
+
+        src = lib.fileset.toSource {
+          root = ./.;
+          fileset = lib.fileset.intersection (lib.fileset.fromSource (lib.sources.cleanSource ./.)) (
+            lib.fileset.unions [
+              ./Cargo.toml
+              ./Cargo.lock
+              ./src
+              ./LICENSE
+            ]
+          );
+        };
+
+        cargoLock.lockFile = ./Cargo.lock;
+
+        meta = {
+          homepage = "https://github.com/catppuccin/catwalk";
+          description = "🚶 Soothing pastel previews for the high-spirited!";
+          license = lib.licenses.mit;
+          mainProgram = "catwalk";
+        };
+      }
+  ) {};
 }