ProjectRepairer: Refactor copy() to take a destination dir

buildCopyDestination() appends objects/pack/ to the remote path
itself, so copy() can only ever populate a destination's pack
directory.

Pass the remote subdirectory in instead, with the pack callers naming
PACK_DIR. The rsync commands produced are unchanged.

Change-Id: I7bf05966dd95b7827864f2ae5a0a39fe9ca050eb