[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.
3.34 (UNRELEASED)
=================
New features
------------
* Various bits of a CubicWeb application configuration can be now be
overridden through environments variables matching configuration option
names prefixed by ``CW_`` (for instance ``CW_BASE_URL``).
* Cubes are now standard Python packages named as ``cubicweb_<cubename>``.
They are not anymore installed in ``<prefix>/share/cubicweb/cubes``. Their
discovery by CubicWeb is handled by a new setuptools entry point
``cubicweb.cubes``. A backward compatibility layer is kept for "legacy"
cubes.