Fixes incorrect generated manifest and generated directory for `index/[...dynamic]` pages
Too much normalizing adding extra `index/` prefix to `index/[...dynamic]` routes which lead to the incorrected generated routes like `.next/server/pages/index/index/index/[...dynamic]`
## Bug
Fixes https://github.com/vercel/customer-issues/issues/146
- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
