[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.

[check-manifest]
ignore =
  debian
  debian/*
  cubicweb.spec
  __pkginfo__.py