[rqlrewrite] Enhance detection of need for Exists node
We actually want one only if this is not a (`And` / `Or`) binary tree of `Not` or
`Exists` nodes, so write a recursive function to tell so.
Related to #17074119
.. _appobject_module:
:mod:`cubicweb.appobject`
=========================
.. automodule:: cubicweb.appobject
.. autoclass:: AppObject
:show-inheritance:
:members: