[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/entities/
+
−usr/lib/python2*/*-packages/cubicweb/ext/
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−usr/share/cubicweb/cubes/
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−usr/lib/python2*/*-packages/cubicweb/*.py