Move finding XML catalogs into a separate setup hook

This is because libxml/libxslt are not the only implementations that
respect $XML_CATALOG_FILES.

Also, look in share/xml for catalogs (in addition to the
now-deprecated xml/dtd and xml/xsl).

pkgs/development/libraries/libxml2/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, zlib, xz, python ? null, pythonSupport ? true }:
+{ stdenv, fetchurl, zlib, xz, python ? null, pythonSupport ? true, findXMLCatalogs }:
 
 assert pythonSupport -> python != null;
 
@@ -22,9 +22,7 @@ stdenv.mkDerivation (rec {
     # RUNPATH for that, leading to undefined references for its users.
     ++ (stdenv.lib.optional stdenv.isFreeBSD xz);
 
-  propagatedBuildInputs = [ zlib ];
-
-  setupHook = ./setup-hook.sh;
+  propagatedBuildInputs = [ zlib findXMLCatalogs ];
 
   passthru = { inherit pythonSupport version; };