Non-composing generators with several input types

[ghost]

Generators which are not composing, but require two input files apepars to be subtly broken. First, if such generator is *passed* two source target the 'convert-to-consumable-types' method will call generators.construct again, for non-obvious reason. Second, if that returns a type that's derived from one of desired source type, we don't detect that, and bypass that target. Finally, we don't check that all required types are present.

[ghost]

Clearing the milestone. Given that I've hit this just once since generators.jam was written, this could not be very important.