bt2: import public names into `__init__.py`

bt2: import public names into `__init__.py`

This patch updates `__init__.py.in` so that it imports exactly the names
which we want to make public (available directly in the `bt2` package).

Most public names which start with `_` exist to be able to test if a
given object is an instance of that type, for example:

    if isinstance(val, bt2._IntegerValue):
        # do something with `val` knowing it's an integer value

`bt2._Error` exists to catch it:

    try:
        # ...
    except bt2._Error as exc:
        # ...

`bt2._User*` names exist to inherit them.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I67c7bed00bdffcd0b1fe63603820ecbe2867ec7a
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1768
Tested-by: jenkins <jenkins@lttng.org>