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).

setup.py

@@ -55,6 +55,18 @@ setup(
         swh-lister=swh.lister.cli:cli
         [swh.cli.subcommands]
         lister=swh.lister.cli:lister
+        [swh.workers]
+        lister.bitbucket=swh.lister.bitbucket:register
+        lister.cgit=swh.lister.cgit:register
+        lister.cran=swh.lister.cran:register
+        lister.debian=swh.lister.debian:register
+        lister.github=swh.lister.github:register
+        lister.gitlab=swh.lister.gitlab:register
+        lister.gnu=swh.lister.gnu:register
+        lister.npm=swh.lister.npm:register
+        lister.packagist=swh.lister.packagist:register
+        lister.phabricator=swh.lister.phabricator:register
+        lister.pypi=swh.lister.pypi:register
     ''',
     classifiers=[
         "Programming Language :: Python :: 3",