To do this we add a stub `cobra.Command` for each installed plugin (only when
invoking `help`, not for normal running).
This requires a function to list all available plugins so that is added here.
Signed-off-by: Ian Campbell <ijc@docker.com>
... and expose. I would like to use this from another site.
This implies also moving (and exposing) the `visitAll` helper.
Unit test them while I'm here.
Signed-off-by: Ian Campbell <ijc@docker.com>