[config] fix the load_site_cubicweb() method for to 'new-style' cubes (closes #16059402)
We first try to load the site_cubicweb module from the cubicweb_<cube> package,
and if it fails, revert back to old cube path.
.. _web_module:
:mod:`cubicweb.web`
===================
.. automodule:: cubicweb.web
Exceptions
----------
.. autoexception:: DirectResponse
.. autoexception:: InvalidSession
.. autoexception:: PublishException
.. autoexception:: LogOut
.. autoexception:: Redirect
.. autoexception:: StatusResponse
.. autoexception:: RequestError
.. autoexception:: NothingToEdit
.. autoexception:: ProcessFormError
.. autoexception:: NotFound
.. autoexception:: RemoteCallFailed