[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

version=3
opts=uversionmangle=s/(rc|a|b|c)/~$1/ \
https://pypi.debian.net/cubicweb/cubicweb-(.+)\.(?:zip|tgz|tbz|txz|(?:tar\.(?:gz|bz2|xz)))