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.

Impacts: This needs a new deployment. A migration script on the scheduler db for that loader (we can avoid impacts on swh-web if we keep the load-archive-files task type, updating only its backend name)

This iterates over the work of the previous mr [1]

[1] !479 (merged)

Refs. swh/infra/sysadm-environment#4906 (closed)