close #511810: bad rql generated when looking for vocabulary for a relation on an entity which doesn't exist (yet) technical details: * add a graph of linked variables in the RRQLExpression instance * modify rql rewriter to remove relations where an unexistant variable and or a variable linked to an unexistant variable is used. * activate this feature in Entity.unrelated_rql if the entity doesn't exists yet

debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/hercule.py usr/lib/PY_VERSION/site-packages/cubicweb