hcp: give a proper error when using conflicting build name

[mogrogan]

It was possible to put the same source 2 times in the build and when using HCP, it would error eventually since they are considered the samebuild from HCP side

Test

packer {
  required_plugins {
    docker = {
      version = ">= 1.0.8"
      source = "github.com/hashicorp/docker"
    }
  }
}

source "docker" "ubuntu-jammy" {
  image  = "ubuntu:jammy"
  commit = true
}

source "docker" "ubuntu-focal" {
  image  = "ubuntu:focal"
  commit = true
}

build {
  name = "jammy-build-block"

  hcp_packer_registry {
    bucket_name = "test-root"
    description = "hcp test with root"
  }
  sources = [
    "source.docker.ubuntu-jammy",
    "source.docker.ubuntu-jammy",
  ]
}

This would give this error:

$ packer build .
Error: Build name conflicts

When using HCP registry, build names are used as identifier. In this case, there
is 2 or more build called jammy-build-block.docker.ubuntu-jammy

[lbajolet-hashicorp]

Overall this looks good to me, I think it maintains existing compatibility, but to be sure, have you run HPATS on those?
I left a couple comments, but nothing blocking, I'll let you address those and come back for another round of review later.