Revert "coord: return Roby::Task as child model if the task model does not respond to find_child"

This reverts commit dd3620b427359884f5c6618234b6522816a1ea6c.
core: fix coupling of all task-level state machines

They were all based on the Proxy object (instead of each their
own private subclass of Proxy) which meant that they were
basically using the same definition
test: fix one dependency relation test