Flatten recipe variable resolution

[Robert Ancell]

Currently Bake allows child recipes to override parent recipe variables. However, some variables can be referenced (e.g. libraries) from all recipes.

I'm not seeing a great use-case for this and it's likely to cause more confusion that it fixes. I propose that there's one flat store of recipes that all Recipe files populate. In this case any variable defined in two recipes would be considered an error.

We may still need a concept of local variables that only relate to the current Recipe file but hopefully that wont be needed.