Fetch::init_fetch's use in QueryState::par_for_each_unchecked_manual is unsound.

## Bevy version
Potentially every one since QueryState::par_for_each_unchecked_manual was introduced.

## What went wrong
The safety invariant on `Fetch::init_fetch` requires that must be legal for the fetch to access all matched components for a query, which is not true for `QueryState::par_for_each_unchecked_manual`, since it's initializing multiple Fetches in parallel and forcing them to access mutually exclusive parts of the query results. Either:

1. ...the safety invariant needs to be more clearly worded and dependent safety invariants updated.
2. ...we should be using a more scoped way to fetch query results.

Given that Fetch implementations are the lowest in the access hierarchy, I'd suggest option 1 and being more stringent in how we pair Fetch initialization and retrieval.