python: standardize intra-bt2 imports

python: standardize intra-bt2 imports

A subsequent patch wants to refer to `object` (the Python type), but it
is shadowed by our `from bt2 import object` import.  It seems like a
good time to standardize how we import intra-bt2 modules, to use the
pattern:

    from bt2 import potato as bt2_potato

Note that I didn't add the bt2_ prefix to the native_bt import, since it
would be a bit redundant.

Change-Id: Icdb4339075a1888463c4f84b292deb272ad49943
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10386
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>