Remove getObjectsDirectory, openPack from base API
These two methods are specific to the FileRepository implementation
and should not be exposed as part of the base Repository API. Now
that PackParser is generic and does not require these two methods
to import a pack stream into a repostiory, it is safe to remove
these and get them out of the public view.
Change-Id: I8990004d08074657f467849dabfdaa7e6674e69a Signed-off-by: Shawn O. Pearce <spearce@spearce.org>