implement listers as plugins

Listers are declared as plugins via the `swh.workers` entry_point.

As such, the registry function is expected to return a dict with the
`task_modules` field (as for generic worker plugins), plus:

- `lister`: the lister class,
- `models`: list of SQLAlchemy models used by this lister,
- `init` (optionnal): hook (callable) used to initialize the lister's state
  (typically, create/initialize the database for this lister).
  If not set, the default implementation creates database tables (after
  optionally having deleted exisintg ones) according to models declared in
  the `models` register field.

There is no need for explicitely add lister task modules in the main
`conftest` module, but any new/extra lister to be tested must be registered
(the tested lister module must be properly installed in the test environment).

Also refactor a bit the cli tools:
- add support for the standard --config-file option at the 'lister' group
  level,
- move the --db-url to the 'lister' group,
- drop the --lister option for the `swh lister db-init` cli tool:
  initializing (especially with --drop-tables) the database for a single
  lister is unreliable, since all tables are created using a sibgle MetaData
  (in the same namespace).

swh/lister/npm/__init__.py

@@ -0,0 +1,13 @@
+# Copyright (C) 2019 the Software Heritage developers
+# License: GNU General Public License version 3, or any later version
+# See top-level LICENSE file for more information
+
+
+def register():
+    from .models import NpmVisitModel, NpmModel
+    from .lister import NpmLister
+
+    return {'models': [NpmVisitModel, NpmModel],
+            'lister': NpmLister,
+            'task_modules': ['%s.tasks' % __name__],
+            }