[layers v2] `service`s names should not be required to be globally unique

For the first pass, I've made it so services are still considered globally unique, so we didn't have to change all of the CLI surface for this next change, as it was getting large. This mirrors the constraint we had in layers v1.

This issue tracks removing some of the code that's requiring uniqueness:
- project/validate (validateParsedConfig), which puts all services into the same map
- ProjectConfig.Services, which also merges everything into a single map

We'll also want to make sure that `up` and `provision` both handle addressing services. There's some possibilities there:
- If a name is globally unambiguous, allow `azd deploy ` and just search all the layers for that name. We could error out if it turns out that name is in more than one layer.