This adds a proper error when we detect conflicting app and page paths.
Fixes: https://vercel.slack.com/archives/C043ANYDB24/p1664678172389449
## Bug
- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have a helpful link attached, see `contributing.md`
Co-authored-by: Sukka <isukkaw@gmail.com>
