feat(nix): flake-parts, first draft

nix/packages/uwulib/default.nix

@@ -0,0 +1,10 @@
+{ inputs, ... }:
+{
+  flake.uwulib = {
+    init = pkgs: {
+      features = import ./features.nix { inherit pkgs inputs; };
+      environment = import ./environment.nix { inherit pkgs inputs; };
+      build = import ./build.nix { inherit pkgs inputs; };
+    };
+  };
+}