Rewrite BinaryComponent::create_targets in a more general way

Instead of assuming a generator-compiler-linker chain, with generators
only coming from the package, it now applies tools until it arrives at
object files, which are then fed to the linker.  This allows global
generator tools and also allows multiple generators to be chained, with
the caveat that if the chain contains more than one step operating on
a directory or a component, the results may be unpredictable.