DfsCachedPack: Add accessor for packfile object

DfsCachedPack: Add accessor for packfile object

In order to support implementations of CachedPackUriProvider (which need
to supply, among other things, the checksum of the packfile
corresponding to a CachedPack), in a004820858 ("UploadPack: support
custom packfile-to-URI mapping", 2019-08-20), the method getCheckSum()
was added to PackIndex. However, there is no way to access the PackIndex
from a DfsCachedPack.

Therefore, add an accessor for the DfsPackFile stored in the
DfsCachedPack. Now, a user who has a DfsCachedPack can use
DfsCachedPack#getPackFile then DfsPackFile#getPackIndex then
PackIndex#getCheckSum to obtain the checksum of a pack.

Change-Id: Ia010c016f6cac0f058ee20eff4c10f57338bfefc
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>