fix: set autoconf package name to babeltrace2

This will help to make sure we are co-installable with babeltrace 1 by
moving the documentation directory to '/usr/share/doc/babeltrace2'.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Change-Id: Ia55d2049967016fc5a00594c928e0f2c4f0e477d
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2858
Reviewed-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>