[FIX] core: harden the legacy `migrations` package matching regex
Since odoo/odoo#122569, we now try to import the `migrations`
sub-package of each module to find upgrade tests.
However, this badly written regex match the OCA module `base_maintenance`,
which generate a RecursionError.
closes odoo/odoo#136282
Signed-off-by:
Christophe Simonis (chs) <chs@odoo.com>
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