[Fiber] Track the Real Fiber for Key Warnings (#29791)

This refactors key warning to happen inline after we've matched a Fiber.
I didn't want to do that originally because it was riskier. But it turns
out to be straightforward enough.

This lets us use that Fiber as the source of the warning which matters
to DevTools because then DevTools can associate it with the right
component after it mounts.

We can also associate the duplicate key warning with this Fiber. That
way we'll get the callsite with the duplicate key on the stack and can
associate this warning with the child that had the duplicate.

I kept the forked DevTools tests because the warning now is counted on
the Child instead of the Parent (18 behavior).

However, this won't be released in 19.0.0 so I only test this in
whatever the next version is.

Doesn't seem worth it to have a test for just the 19.0.0 behavior.