tasks: normalize the url argument name of most lister

Since all the listing tasks accepts an url as first argument (whatever the
argument name is), it makes sense to use a simple common argument name for
this. I've chosen 'url' instead of api_baseurl/forge_url/url.

Also kill now useless `new_lister()` functions.

swh/lister/cli.py

@@ -63,7 +63,7 @@ def get_lister(lister_name, db_url=None, **conf):
             (lister_name, SUPPORTED_LISTERS))
     if db_url:
         conf['lister'] = {'cls': 'local', 'args': {'db': db_url}}
-    # To allow api_baseurl override per lister
+
     registry_entry = LISTERS[lister_name].load()()
     lister_cls = registry_entry['lister']
     lister = lister_cls(override_config=conf)