bt2: rename _FieldPath to _FieldPathConst

The Babeltrace API only returns const field_path objects, so from the
point of view of the user, it's always const.  Rename the Python object
to _FieldPathConst to reflect this.  If the API ever gives access to a
non-const version of field_path, then we can introduce a _FieldPath
type again.

Change-Id: I74a2889db48d570b4b6fbc28e43f98226dc5a053
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2055
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins <jenkins@lttng.org>