Auto merge of #78780 - cjgillot:req, r=Mark-Simulacrum

Auto merge of #78780 - cjgillot:req, r=Mark-Simulacrum

Refactor query forcing

The control flow in those functions was very complex, with several layers of continuations.

I tried to simplify the implementation, while keeping essentially the same logic.
Now, all code paths go through `try_execute_query` for the actual query execution.
Communication with the `dep_graph` and the live caches are the only difference between query getting/ensuring/forcing.