[devctl] make i18n custom message extractors work with legacy layouts distname needs to be "cubicweb_<cubename>", even with legacy layouts for pkg.load_entry_point() to work closes #16272177

usr/lib/python2*/*-packages/cubicweb/web
usr/lib/python2*/*-packages/cubicweb/wsgi
usr/share/cubicweb/cubes/shared/data
usr/share/cubicweb/cubes/shared/wdoc