Split out packfile handling from ObjectDirectory

Split out packfile handling from ObjectDirectory

The ObjectDirectory class manages the interactions for the entire object
database, this includes loose objects, packfiles, alternates, and
shallow commits. To help reduce the complexity of this class, abstract
some of the packfile specific details into a class which understands
just this, leaving the ObjectDirectory to focus more on the interactions
between the different mechanisms.

Change-Id: I5cc87b964434b0afa860b3fe23867a77b3c3a4f2
Signed-off-by: Martin Fick <mfick@codeaurora.org>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>