When hydra evaluates a jobset, 'package-transitive-supported-systems' is
called for every package+system combination.  Each of these calls
traverses the tree of inputs, but without eliminating duplicate
transitive-inputs.  In other words, the amount of time spent is
proportional not to the number of transitive-inputs, but the number of
_paths_ to all transitive-inputs.

This patch memoizes 'package-transitive-supported-systems', so that the
total time to apply it to all packages is O(N).

     Mark