Harshank Bansal provided the correct answer. If you want to post your comment as an answer I will mark it correct.

Adding those imports in the global.xml made the XMLs visible to each other.

Unfortunately for us, once the flows were all universally visible, we learned that a universal logging component that we have in all of our Mule 3 projects is very much incompatible with Mule 4 so we may just end up doing a ground up re-write after all.