Use rewritten pathname for implicit cache tags (#88732)
When a page is accessed via a rewrite, use the destination path (not the source path) when computing implicit tags. This ensures that calling `revalidatePath('/dest')` correctly invalidates cache entries for pages rewritten to that destination.
Fixes `'use cache'` and fetch cache with rewrites. ISR page cache was not affected because it already uses the rewritten pathname as the cache key (via `resolvedUrlPathname` in `base-server.ts`).
