[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>