[WIP] import-url: use dvc-data `index.save()` for fetching imports

[pmrowla]

• [ ] ❗ I have followed the Contributing to DVC checklist.

• [ ] 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here.

Thank you for the contribution - we'll try to review it as soon as possible. 🙏