cli: register the 'lister' cli subcommand

also add a cli group named 'lister' for the sake of consistency with
other swh packages and rename the command as 'db-init', like:

  swh lister db-init LISTER [...]

setup.py

@@ -54,6 +54,8 @@ setup(
     entry_points='''
         [console_scripts]
         swh-lister=swh.lister.cli:cli
+        [swh.cli.subcommands]
+        lister=swh.lister.cli:lister
     ''',
     classifiers=[
         "Programming Language :: Python :: 3",

swh/lister/cli.py

@@ -6,6 +6,8 @@
 import logging
 import click
 
+from swh.core.cli import CONTEXT_SETTINGS
+
 
 logger = logging.getLogger(__name__)
 
@@ -13,7 +15,14 @@ SUPPORTED_LISTERS = ['github', 'gitlab', 'bitbucket', 'debian', 'pypi',
                      'npm', 'phabricator']
 
 
-@click.command()
+@click.group(name='lister', context_settings=CONTEXT_SETTINGS)
+@click.pass_context
+def lister(ctx):
+    '''Software Heritage Lister tools.'''
+    pass
+
+
+@lister.command(name='db-init', context_settings=CONTEXT_SETTINGS)
 @click.option(
     '--db-url', '-d', default='postgres:///lister-gitlab.com',
     help='SQLAlchemy DB URL; see '
@@ -22,8 +31,9 @@ SUPPORTED_LISTERS = ['github', 'gitlab', 'bitbucket', 'debian', 'pypi',
                 type=click.Choice(SUPPORTED_LISTERS + ['all']))
 @click.option('--drop-tables', '-D', is_flag=True, default=False,
               help='Drop tables before creating the database schema')
-def cli(db_url, listers, drop_tables):
-    """Initialize db model according to lister.
+@click.pass_context
+def cli(ctx, db_url, listers, drop_tables):
+    """Initialize the database model for given listers.
 
     """
     override_conf = {