Prevent creation of broken transform rules. refs #3462
This raises validation errors (with links to the colliding rule) for the following cases:
* Overlapping rules. You can't make a transform "red"->"blue" if you already have a "red"->"green"
* Transitory rules. You can't make a transform "red"->"blue" if you have "blue"->"green".
  Instead, you should transform from "red"->"green"

We still need to keep the logic only applying one rule per tag, because "contains" rules are considerably
more complex (starting with contains "Biden"->"Vice President Joe Biden", which would be recursive.)