sim: allow forward dependencies in checkpoint upgraders

sim: allow forward dependencies in checkpoint upgraders

The notion of forward dependencies is just expressing the same
dependency but at the other end of the dependency edge, i.e. at
the dependee rather than the depender.  As there is no more
'power' here, it's strictly a convenience feature for handling
dependencies with tags that are not in the upstream repository.

Change-Id: Ic7c68de6aff4094aaa12de62cdf690a5dc65ccb5
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>