cli: do not use --component in help command, only use leftover argument

cli: do not use --component in help command, only use leftover argument

Options should be used for optional arguments. In the case of
`babeltrace help`, the component class specification is mandatory when
you don't want help about a plugin, so use the first positional
"leftover" argument.

First try the TYPE.PLUGIN.COMPCLS form, then fallback to PLUGIN if it's
invalid.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>