tasks: Ensure tasks can be scheduled

This:
- Adds a generic test on all tasks modules of the package to ensure tasks and lister
  visit type are ok so scheduling can happen.
- Opens an utility function so we can use it on other loader module
- fixes the archive loader which was misnamed and adapts accordingly its task and its
  loader's visit types.

This now needs a new deployment and a migration script on the scheduler db for that
loader.

Refs. swh/infra/sysadm-environment#4906