refactor(nix): rename lib.nix to meta.nix and add flake.root option

flake.nix

@@ -56,6 +56,7 @@
       (inputs.import-tree ./nix)
       // {
         systems = import inputs.systems;
+        flake.root = ./.;
       }
     );
 }

nix/flake-parts/meta.nix

@@ -1,7 +1,7 @@
 { lib, inputs, ... }:
 let
   inherit (lib.options) mkOption;
-  inherit (lib.types) lazyAttrsOf raw;
+  inherit (lib.types) path lazyAttrsOf raw;
   inherit (inputs.flake-parts.lib) mkSubmoduleOptions;
 in
 {
@@ -10,5 +10,9 @@ in
       type = lazyAttrsOf raw;
       default = { };
     };
+    root = mkOption {
+      type = path;
+      default = "";
+    };
   };
 }