Add relative app dir field (#46393)
This adds a relative app dir field to the required files manifest so
that we don't rely on absolute paths from a cached build since the cache
can be restored in a separate context where the value no longer applies.
x-ref: https://github.com/vercel/next.js/pull/45864
## Bug
- [ ] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have a helpful link attached, see
[`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)
