Implement better dependency tracking for ipynb.fs.defs

Right now, we only include assignments where the LHS is all caps. However, LHS all caps might have dependencies on RHS that's *not* all caps. These will not be evaluated, causing strange amounts of confusion.

Instead, we should attempt to track the dependencies on the RHS of such assignments, and eval those too (recursively). We should do this in such a way that the rules are very clear to explain to people.

This should still only be done for top level imports though.