plugins: add support for scheduler's task-type declaration

Add a new register-task-types cli that will create missing task-type entries in the
scheduler according to:

• only create missing task-types (do not update them), but check that the backend_name field is consistent,
• each SWHTask-based task declared in a module listed in the 'task_modules' plugin registry field will be checked and added if needed; tasks which name start wit an underscore will not be added,
• added task-type will have:
  • the 'type' field is derived from the task's function name (with underscores replaced with dashes),
  • the description field is the first line of that function's docstring,
  • default values as provided by the swh.lister.cli.DEFAULT_TASK_TYPE (with a simple pattern matching to have decent default values for full/incremental tasks),
  • these default values can be overloaded via the 'task_type' plugin registry entry.

For this, we had to rename all tasks names (eg. cran_lister -> list_cran).

Comes with some tests.