Simplify handling of union fixtures in the engine and remove the associated warning

[smarie]

Currently an annoying warning is issued when fixture unions are used ("WARNING the new order is not taken into account !!").

This is due to the fact that we keep in memory all the tree built during creation of the alternate fixture closures. But this tree is actually not useful anymore after construction: we should flatten it into a list of alternate closures (one per tree leaf).

Once we do this, the rest of the plugin code can be simplified greatly and the reordering can be done easily on each sub-closure (by only taking into account the relevant names for this sub-closure)