[turbopack] Simplify up and dowcasts (#83456)
## What
Casting `ResolvedVc<Box<dyn Module>>` to `ResolvedVc<Box<dyn Module>>` is confusing, so we should reject it.
Ditto for no op `upcast`s.
This PR makes such calls illegal and eliminates them from the codebase.
## How
Introduce a new trait `UpcastStrict` that is only implemented for types that extend or implement different traits. This way we can say it is only valid to try to downcast if the target type is `UpcastStrict`.
Then modify the type constraints on our casting utilties to require `UpcastStrict` instead of `Upcast`
In the case of `Vc::upcast` i also introduced a `Vc::upcast_non_strict` to help with writing generic code or extension traits.
## Why?
I got very confused by some `try_downcast` calls that turned out to be no-ops.
Of course this might make some type theorists unhappy, and I don't disagree. Downcasting X->X is a legitimate operation. The ideal solution here would be a linter rule, but there doesn't appear to be any way to implement that.
