[devtools] Handle i18ncube command for "cubes as packages" And add a test so that both layouts are tested. Move the test cube into a package. Related to #13001466.

usr/bin/cubicweb-ctl usr/bin/
usr/lib/python2*/*-packages/cubicweb/cwctl.py
../cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl