refactor(flake): optimize structure

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

modules/common.nix

@@ -0,0 +1,12 @@
+{ inputs, ... }:
+
+{
+  imports = [
+    inputs.self.nixosModules.flakeDefaults
+    inputs.agenix.nixosModules.default
+  ];
+
+  nixpkgs.overlays = [ inputs.self.overlays.default ];
+
+  environment.systemPackages = [ inputs.agenix.packages.x86_64-linux.default ];
+}