Reduce the number of Makefiles in 'src/plugins/text'

Continue the effort started by Simon to reduce the overall number of
Makefiles to allow for faster parallel builds. Also reduce the number of
convenience libraries for faster linking.

Change-Id: I1e706eb0bb5d52d0172ab6d0773f861865562158
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/11154
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>