PackWriter: use lib.ObjectIdSet to avoid wrapper

PackWriter: use lib.ObjectIdSet to avoid wrapper

Hoist ObjectIdSet up to lib as part of the public API and add
the interface to some common types like PackIndex and JGit custom
ObjectId map types.  This cleans up wrapper code in a number of
places by allowing direct use of the types as an ObjectIdSet.

Future commits can now rely on ObjectIdSet as a simple read-only
type to check a set of objects from a number of storage options.

Change-Id: Ib62b062421d475bd52abd6c84a73916ef36e084b