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

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

DfsPackFile assumes the indices are in pack streams, but we would like
to consider other formats and storage. Currently, the local ref in the
DfsPackFile to the index is set in the cache loading callback, which
prevents abstracting the loading.

Reorganize the code so: the loadPackIndex function just parses the bytes
returning a reference and the caller sets the loaded index in the local
ref and DfsBlockCache.

We will follow this pattern with other indices in follow-up
changes. Note that although DfsPackFile is used only in one thread,
the loading in DfsBlockCache can happen from multiple threads
concurrently and we want to keep only one ref around.

Change-Id: Ie99de6c80784225484c0f99c51caa44c6a372c45