[pkg] Warn about "develop" command not installing legacy cubes directory Not sure we can do better than a warning, which is better than nothing anyways. Closes #15785635.

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