Refactor ContentSources to RouteTree (#51660)
### What?
This fixes a performance problem when many pages are involved.
fixes WEB-1067
see also https://github.com/vercel/turbo/pull/5360
### Turbopack Changes
* https://github.com/vercel/turbo/pull/5416
* https://github.com/vercel/turbo/pull/5360
