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.

swh/lister/gnu/tasks.py

@@ -8,10 +8,11 @@ from .lister import GNULister
 
 
 @app.task(name=__name__ + '.GNUListerTask')
-def gnu_lister(**lister_args):
+def list_gnu_full(**lister_args):
+    'List lister for the GNU source code archive'
     GNULister(**lister_args).run()
 
 
 @app.task(name=__name__ + '.ping')
-def ping():
+def _ping():
     return 'OK'