DfsPackFile: make #getReverseIdx public

DfsPackFile: make #getReverseIdx public

The DfsPackFile#getReverseIdx method, which wraps creating a
PackReverseIndex in caching, was package-private. This caused
implementations on top of DfsPackFile to directly instantiate a
PackReverseIndex in cases where it would benefit from caching.

Instead, make #getReverseIdx public so that the caching logic can be
reused by implementations where appropriate.

Change-Id: I4553e514a4ac320bfe2455c00023343ad97f9d15
Signed-off-by: Anna Papitto <annapapitto@google.com>