doc-support: don't expose locationsXml

nothing except function docs uses this, so we need not expose it. we'll
be reworking this entire section of the build anyway, with the target of
breaking up doc-support as it is now.

doc/doc-support/lib-function-docs.nix

@@ -1,8 +1,13 @@
 # Generates the documentation for library functions via nixdoc.
 
-{ pkgs, locationsXml, libsets }:
+{ pkgs, nixpkgs, libsets }:
 
-with pkgs; stdenv.mkDerivation {
+with pkgs;
+
+let
+  locationsXml = import ./lib-function-locations.nix { inherit pkgs nixpkgs libsets; };
+in
+stdenv.mkDerivation {
   name = "nixpkgs-lib-docs";
   src = ../../lib;