inconsistent behavior of dynamic multiplex rules

Dynamic multiplex rules, like those in the example for QBS-640, have at least two problems:

• the outputArtifacts script sees only inputs that are out of date. It should see all inputs this rule is interested in.
• touching one .in file in the example results in rebuilding all outputs of that rule. This is a result of the fact that every output is connected to every input in a multiplex rule.