[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.

Release notes
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.. toctree::
    :maxdepth: 1

    3.23
    3.22
    3.21
    3.20
    3.19
    3.18
    3.17
    3.16
    3.15
    3.14