Add support for multiple "src" arguments for PathSetTask.

Used by answer distribution, course enrollment, user location,
and enrollment reports.

This is the single commit extracted from the synthetic event PR that allows the daily course enrollment job to run with synthetic events.
What is `self.src` is not a list? Should we add a check in case someone forgets to put `is_list=True` in the parameter? Alternatively, if it is not a list, then we can put it in one before iterating.