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.