Recent changes in swh-deposit codebase introduce cascading import of
swh.deposit.models.Deposit when importing DepositChecker class from
the swh.deposit.loader.checker module.

Importing swh.deposit.models.Deposit requires django to be properly
initialized which is usually not the case when the celery task modules
are imported through the swh_scheduler_celery_worker pytest fixture,
so numerous tests related to scheduler tasks in various swh packages
were failing.

So move import of DepositChecker class from top level to the task
function to avoid such side effect.