clean up pack index handling a bit

clean up pack index handling a bit

Especially with the new index format to come, it is more appropriate
to encapsulate more into check_packed_git_idx() and assume less of the
index format in struct packed_git.

To that effect, the index_base is renamed to index_data with void * type
so it is not used directly but other pointers initialized with it. This
allows for a couple pointer cast removal, as well as providing a better
generic name to grep for when adding support for new index versions or
formats.

And index_data is declared const too while at it.

(based on commit 4287307833a7c67b09973fc1023311e473f830b2)

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>