Further generators simplications.
1. If when generating something, we find more that one suitable generators,
run them and more then one return something, immediately report ambiguity.
Don't care if the produced targets are the same. This is better that
running several generators all the time, performance wise.
2. Remove the notion of 'intermediate' virtual-targets. IIRC, they were used
to prevent staging of RSP files, and we don't stage them anyway now.
[SVN r29491]
Vladimir Prus