tasks: Simplify implementation and make visit_date parameter optional

Recent changes in swh-scheduler add new parameters to the celery tasks
produced from swh.scheduler.model.ListedOrigin instances.

So ensure to handle any new parameters by not hardcoding the expected
ones in task signatures.

Rename date parameter to visit_date in from disk loader tasks and
make it non mandatory.

Add new tests checking task parameters produced from ListedOrigin
instances do no raise error when attempting to create a git loader.

Related to T4187