Turbopack: deduplicate Pages Router app chunks (#97664)
Previously, Turbopack treated `_app` and Pages router pages as
completely separate.
So using code both in `_app` and `pages/foo.tsx` would lead to a lot of
duplicated code to be loaded at runtime.
But in reality, `_app` is always loaded for Pages, so we can thread the
availability info and skip chunking modules that were already loaded by
`_app`.
Recreation of https://github.com/vercel/next.js/pull/97549
Co-authored-by: vercel-fleet-prod[bot] <318278635+vercel-fleet-prod[bot]@users.noreply.github.com>
