bindings/python/bt2: Make the bt2 package importable

bindings/python/bt2: Make the bt2 package importable

This patch changes the minimum necessary to make the bt2 package
importable.  In other words, for this command to succeed:

    $ tests/utils/test_python_bt2_env python3 -c "import bt2"

In some cases, I've replaced some references to concepts that don't
exist with the new concepts (e.g. some NOTIFICATION_TYPE_* to
MESSAGE_TYPE_*).  In other cases, (e.g. fields.py and field_types.py),
it was easier to remove the code, knowing we'll add it back when
tackling the tests that require these files.

After this milestone, we'll be able to at least run some tests that
import the bt2 package, so we'll be able to make sure we don't regress
things while we fix others.

Change-Id: I930d5a4dd129777cb40c607670e27002bb727e39
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.gerrithub.io/c/eepp/babeltrace/+/452154
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: Philippe Proulx <eeppeliteloop@gmail.com>