emacsPackagesFor: Internalise trivialBuild/melpaBuild

I want less strangeness that's not obviously overrideable.

pkgs/top-level/all-packages.nix

@@ -21952,14 +21952,6 @@ in
     inherit fetchFromGitHub fetchurl;
     inherit emacs texinfo makeWrapper runCommand writeText;
     inherit (xorg) lndir;
-
-    trivialBuild = callPackage ../build-support/emacs/trivial.nix {
-      inherit emacs;
-    };
-
-    melpaBuild = callPackage ../build-support/emacs/melpa.nix {
-      inherit emacs;
-    };
   };
 
   inherit (gnome3) empathy;

pkgs/top-level/emacs-packages.nix

@@ -35,14 +35,20 @@
 { lib, newScope, stdenv, fetchurl, fetchFromGitHub, runCommand, writeText
 
 , emacs, texinfo, lndir, makeWrapper
-, trivialBuild
-, melpaBuild
 
 , pkgs
 }:
 
 let
 
+  trivialBuild = pkgs.callPackage ../build-support/emacs/trivial.nix {
+    inherit emacs;
+  };
+
+  melpaBuild = pkgs.callPackage ../build-support/emacs/melpa.nix {
+    inherit emacs;
+  };
+
   mkElpaPackages = import ../applications/editors/emacs-modes/elpa-packages.nix {
     inherit lib stdenv texinfo writeText;
   };