src.ctf.fs: make ctf_fs_ds_file_group::index a ctf_fs_ds_index::UP

src.ctf.fs: make ctf_fs_ds_file_group::index a ctf_fs_ds_index::UP

Change that field to be a unique_ptr.  Adjust some functions in fs.cpp
around that.

Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Change-Id: Ie2cdfe0b81f1d817093de74faf6c12ef2e6169a1
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8252
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12290