So far, in order to get the pack directory it was necessary to resolve
it from the object directory. This resolution is already done when
creating the object directory, so simplify the call by just adding a
getter to the pack directory.
Change-Id: I69e783141dc6739024e8b3d5acc30843edd651a7
Signed-off-by: Hector Caballero <hector.caballero@ericsson.com>