Add the format.PackIndexEntry interface

The format.PackIndexEntry interface represents an entry within a pack index. It
provides methods that return information typically provided by pack index
entries, such as the SHA-1 of an object or the offset of said object in the
corresponding index file.
Add PackIndexEntry-based methods to the format.PackIndex interface

The types format.PackIndexV1 and format.PackIndexV2 have been updated to
continue to implement the format.PackIndex interface.