PackReverseIndex is a concrete class whose implementation is computed
from a pack's forward index. Callers which have a reverse index file may
want to use an implementation that is file-based instead.
Generalize PackReverseIndex into an interface without
implementation-specific logic and separate out the logic for the
computed implementation into a new concrete class.
Change-Id: I98d9835363c5e1c8c3c11a81b0761af3cdeaa41a
Signed-off-by: Anna Papitto <annapapitto@google.com>
8e61971620