refactor(flake): optimize structure

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

lib/default.nix

@@ -0,0 +1,21 @@
+{ inputs, ... }:
+
+let createUser' = import ./createUser.nix;
+in {
+  flake = {
+    lib = {
+      createSystem = hostName:
+        { hardwareConfig, config }:
+        ({ pkgs, lib, ... }: {
+          networking.hostName = hostName;
+
+          imports = [ ../modules/common.nix hardwareConfig config ];
+        });
+      createUser = name: args:
+        ({ pkgs, ... }@args2:
+          (createUser' name args)
+          ({ inherit (inputs) home-manager; } // args2));
+      createMediaGroup = _: { users.groups.media.gid = 600; };
+    };
+  };
+}