[sources management] properly set __linkto, closes #1922970
when clicking on 'add mapping part' on an external source view, one expect to have source properly selected in form
#! /bin/sh -e#if [ "$1" = configure ]; then if which install-docs >/dev/null 2>&1; then install-docs -i /usr/share/doc-base/cubicweb-doc fifi#DEBHELPER#exit 0