compute extrapath, necessary for proper schema module detection when CW_CUBES_PATH is set
"""server side objects:organization: Logilab:copyright: 2001-2009 LOGILAB S.A. (Paris, FRANCE), license is LGPL v2.:contact: http://www.logilab.fr/ -- mailto:contact@logilab.fr:license: GNU Lesser General Public License, v2.1 - http://www.gnu.org/licenses"""