seed prefetch cache with initial page (#61535)
### What
When navigating back to a page that you had already loaded, it currently
results in a prefetch cache miss and will re-trigger any data
fetching/loading despite it being available.
### Why
When creating the initial router state, the prefetch cache is
initialized to an empty map.
### How
This uses the `initialTree` passed from the server to seed the cache for
that route with flight data.
Closes NEXT-2001
