python: remove internal `import bt2` imports

python: remove internal `import bt2` imports

Standardize the internal imports to import specific modules like this:

    from bt2 import foo as bt2_foo

... instead of importing the complete bt2 package.

Change-Id: I32eb223a8bcfe841020a2aafb976f121dadd8680
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/10388
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>