[rql rewrite] move some code from querier to rqlrewrite where it makes more sense.
Also, make some minor cleanup/refactoring on the way and try to enhance
docstrings.
Relates to #3013535
#! /bin/sh -ecase "$1" in remove) if [ -x "/etc/init.d/cubicweb-ctl" ]; then invoke-rc.d cubicweb-ctl stop || true fi ;;esac#DEBHELPER#exit 0