DfsPackFile: Do not set local reverse index ref from cache callback

DfsPackFile: Do not set local reverse index ref from cache callback

The DfsBlockCache loading callback sets the local reference to the
index in the DfsPackFile. This prevents abstracting the loading to
implement it over multiple backends.

Reorg the code so the loadReverseIndex do only the loading, the caller
sets it into DfsBlockCache and the external code sets the local
reference in DfsPackFile.

This is the same pattern we did with the PackIndex in the parent
commit.

Change-Id: I3a395b347965fa7b0e5a3398c4da311dc11c58a1