lib: move code only used by create_event() to `message/event.c`

lib: move code only used by create_event() to `message/event.c`

Those functions, bt_event_set_packet() and bt_event_set_stream(), used
to exist in the Babeltrace 1 to 2 transition. But now it seems like
they're internal and only used by the also internal create_event().

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: Id9087e802af3aca76de6dd89abf73b3d75c848ea
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3429