refactor shell out into nix/ and add nix package

flake.nix

@@ -13,21 +13,17 @@
     devShells = forEachSystem (
       system: let
         pkgs = pkgsForEach.${system};
-      in {
-        default = pkgs.mkShell {
-          name = "osu++";
-
-          nativeBuildInputs = with pkgs; [
-            gcc
-            gnumake
-            # for clang-format
-            clang-tools
-          ];
-
-          buildInputs = with pkgs; [
-            sfml
-          ];
-        };
+      in rec {
+        osuplusplus = import ./nix/shell.nix {inherit pkgs;};
+        default = osuplusplus;
+      }
+    );
+    packages = forEachSystem (
+      system: let
+        pkgs = pkgsForEach.${system};
+      in rec {
+        osuplusplus = import ./nix/default.nix {inherit (pkgs) lib stdenv sfml;};
+        default = osuplusplus;
       }
     );
   };

nix/default.nix

@@ -0,0 +1,21 @@
+{
+  lib,
+  stdenv,
+  sfml,
+}:
+stdenv.mkDerivation {
+  pname = "osu++";
+  version = "0.1.0";
+
+  src = ../.;
+
+  buildInputs = [
+    sfml
+  ];
+
+  meta = with lib; {
+    homepage = "https://git.jacekpoz.pl/jacekpoz/osuplusplus";
+    description = "An osu client written in C++ using SMFL.";
+    license = licenses.agpl3;
+  };
+}

nix/shell.nix

@@ -0,0 +1,15 @@
+{pkgs}:
+pkgs.mkShell {
+  name = "osu++";
+
+  nativeBuildInputs = with pkgs; [
+    gcc
+    gnumake
+    # for clang-format
+    clang-tools
+  ];
+
+  buildInputs = with pkgs; [
+    sfml
+  ];
+}