Detecting system order ambiguities requires a specific world

# Problem

Detecting system order ambiguities should be entirely agnostic to the world that we're running on.

However, schedules (well, `SystemStage` for now) must be initialized on a specific `World` (taking `&mut World`) before we can check for ambiguities, or the whole thing panics.

## Proposed solution

1. Clearly differentiate between "factual conflicts" and "hypothetical conflicts" on data access (#3119).
2. Ensure that the former requires a specific world, while the latter does not.
3. Compute system execution order ambiguities using only hypothetical conflicts.

## Additional context

Discovered in #4299, as this makes exposing useful APIs and writing tests substantially harder than it should.

#1481 needs to take this design into account: we'll need to pass in the archetype invariants to the methods that compute hypothetical invariants