stdenv/check-meta: move to within lib/

This is going to be used in lib.systems, which isn't allowed to import
from outside the lib folder. We don't actually expose this through lib,
though.

lib/meta-types.nix

@@ -1,6 +1,7 @@
 { lib }:
 # Simple internal type checks for meta.
-# This file is not a stable interface and may be changed arbitrarily.
+# This file is only intended for internal nixpkgs use.
+# It is not a stable interface and may be changed arbitrarily.
 #
 # TODO: add a method to the module system types
 #       see https://github.com/NixOS/nixpkgs/pull/273935#issuecomment-1854173100

pkgs/stdenv/generic/check-meta.nix

@@ -300,7 +300,7 @@ let
 
   metaType =
     let
-      types = import ./meta-types.nix { inherit lib; };
+      types = import ../../../lib/meta-types.nix { inherit lib; };
       inherit (types)
         str
         union

pkgs/stdenv/generic/problems.nix

@@ -242,7 +242,7 @@ rec {
   # The type for meta.problems
   problemsType =
     let
-      types = import ./meta-types.nix { inherit lib; };
+      types = import ../../../lib/meta-types.nix { inherit lib; };
       inherit (types)
         str
         listOf