Improvements for get-difficulty (#7720)

Improvements for get-difficulty (#7720)

This makes 2 improvements for get-difficulty:
(1) Ensures that we recompute relevance info for lemmas sent at STANDARD effort.
(2) An open proof error was reported by Certora using --produce-difficulty.
This makes get-difficulty more robust by ensuring that lemmas are not marked as having a difficulty. It also minimizes the cases where we mark lemmas as having a difficulty internally.