refactor(flake): optimize structure

- get rid of some `rec`s
- move lib & overlays into flake modules

overlays.nix

@@ -0,0 +1,16 @@
+{ inputs, ... }:
+
+{
+  flake = {
+    overlays.default = final: prev: {
+      unstable = import inputs.nixpkgs-unstable {
+        inherit (prev) system;
+        config.allowUnfree = true;
+      };
+      inherit (inputs.fh.packages.${prev.system}) fh;
+      inherit (inputs.self.packages.${prev.system}) deconz brlaser;
+      vimPlugins = prev.vimPlugins
+        // final.callPackage ./pkgs/vim-plugins { inherit inputs; };
+    };
+  };
+}