`Item` and `fetch` should be moved to `QueryData` from `WorldQuery`

[alice-i-cecile]

It's kind of weird that QueryFilters even have an Item. I think it's mostly because they used to be a single trait, and the change to separate them tried to minimize changes.

Would be more clear if we moved Item and fetch from WorldQuery to QueryData, and then did all the filtering in filter_fetch? (Not in this PR, of course.) It doesn't look like fetch is ever called on a QueryFilter outside its own implementation, so that might work.

Originally posted by @chescock in #17654 (comment)