[layers v2] Rework `azd add` to land AVMs under infra instead of resources

Part of #9765.

Once we get support for layered provisioning v1.5's format, we can change `azd add` so AVM-backed resources are written into `infra` (as a layer) rather than into the legacy `resources:` model, which was unfortunately not compatible with `infra`.

As part of this we can remove the `resources:` code — this would be a complete replacement, now using layered provisioning. `resources:` was never shipped as GA, so this should have a small impact. We can verify that with telemetry, if need be.

Start with the `azd add` command and trace how AVM-backed resources are written to the legacy `resources:` model. Review the layered provisioning v1.5 work from #9765 and telemetry for `resources:` usage; done means AVM-backed resources land under `infra` as a layer and the obsolete `resources:` code is removed.

Written by the indexing model from the issue text.