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.