[cwconfig] Look for migration scripts directory relatively to code Thus getting rid of the last piece of data in "shared" directory. As a consequence, we can drop data_files from __pkginfo__.py. Related to #17132990.

Release notes
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.. toctree::
    :maxdepth: 1

    3.25
    3.24
    3.23
    3.22
    3.21
    3.20
    3.19
    3.18
    3.17
    3.16
    3.15
    3.14