splice.nix: improve performance with early cut-off (#350295)

pkgs/top-level/splice.nix

@@ -128,6 +128,15 @@ let
     "packages"
   ];
 
+  packagesWithXorg = pkgs // builtins.removeAttrs pkgs.xorg [
+    "callPackage"
+    "newScope"
+    "overrideScope"
+    "packages"
+  ];
+
+  pkgsForCall = if actuallySplice then splicedPackagesWithXorg else packagesWithXorg;
+
 in
 
 {
@@ -138,9 +147,9 @@ in
   # `newScope' for sets of packages in `pkgs' (see e.g. `gnome' below).
   callPackage = pkgs.newScope { };
 
-  callPackages = lib.callPackagesWith splicedPackagesWithXorg;
+  callPackages = lib.callPackagesWith pkgsForCall;
 
-  newScope = extra: lib.callPackageWith (splicedPackagesWithXorg // extra);
+  newScope = extra: lib.callPackageWith (pkgsForCall // extra);
 
   # prefill 2 fields of the function for convenience
   makeScopeWithSplicing = lib.makeScopeWithSplicing splicePackages pkgs.newScope;
@@ -169,5 +178,5 @@ in
 
   # Haskell package sets need this because they reimplement their own
   # `newScope`.
-  __splicedPackages = splicedPackages // { recurseForDerivations = false; };
+  __splicedPackages = if actuallySplice then splicedPackages // { recurseForDerivations = false; } else pkgs;
 }