Introduce a new way to search for `not-found` component that based on
the request pathname and current loader tree of that route. And we
search the proper not-found in the finall catch closure of app
rendering, so that we don't have to pass down the root layout to
app-router to create the extra error boundary.
This ensures the root layout doesn't have duplicated rendering for
normal requests
Fixes NEXT-1220
Fixes#49115
9313c51bc4