Do not automagically add click cmd from swh.cli.subcommands

the idea is now to be a bit more explicit, and expect thoses commands
to have the main `swh` click group as parent.

Then the swh.cli.subcommands entrypoint hooks only have to load the
declared cli modules.

So we now expect to:

- use swh.core.cli.swh as parent group when declaring
  sub-groups/commands in a swh package.
- the swh.cli.subcommand entrypoint to only refer the module rather than
  the click group/command, ie. have:

  '''
    [swh.cli.subcommands]
    name = swh.path.to.cli_module
  '''

  instead of:

  '''
    [swh.cli.subcommands]
    name = swh.path.to.cli_module:click_command
  '''

A backward compatilibility mech is provided: if the loaded entrypoint
object is indeed a click command, auto add it to the main swh group.

setup.py

@@ -67,8 +67,7 @@ setup(
         swh=swh.core.cli:main
         swh-db-init=swh.core.cli.db:db_init
         [swh.cli.subcommands]
-        db=swh.core.cli.db:db
-        db-init=swh.core.cli.db:db_init
+        db=swh.core.cli.db
         [pytest11]
         pytest_swh_core = swh.core.pytest_plugin
     """,

swh/core/cli/__init__.py

@@ -5,6 +5,7 @@
 
 import logging
 import logging.config
+import warnings
 
 import click
 import pkg_resources
@@ -117,7 +118,17 @@ def main():
     for entry_point in pkg_resources.iter_entry_points("swh.cli.subcommands"):
         try:
             cmd = entry_point.load()
-            swh.add_command(cmd, name=entry_point.name)
+            if isinstance(cmd, click.BaseCommand):
+                # for BW compat, auto add click commands
+                warnings.warn(
+                    f"{entry_point.name}: automagic addition of click commands "
+                    f"to the main swh group is deprecated",
+                    DeprecationWarning,
+                )
+                swh.add_command(cmd, name=entry_point.name)
+            # otherwise it's expected to be a module which has been loaded
+            # it's the responsibility of the click commands/groups in this
+            # module to transitively have the main swh group as parent.
         except Exception as e:
             logger.warning("Could not load subcommand %s: %s", entry_point.name, str(e))