Currenly, we treat all modified fields as opaque computations, and we use the dependencies for the entire stage as dependencies for the field (superset).

With the describeTransformation interface from SERVER-119380, we can now query the underlying expression for the path.

This means we can call getDependencies and report dependencies just for that expression. We might want to re-run the benchmarks to see if the additional deps reporting has an impact. We can special case expressions such as constant expression (no deps) if so.