Implement and test multi-guest support for artifact plugin

[LecrisUT]

This primarily becomes a problem after #5089 where we need to feed in the specific NVR package in the artifact to install. Let's say we have

provision:
  - name: fedora-host
    how: container
    image: fedora
  - name: debian-host
    how: container
    image: debian
prepare:
  - how: artifact
    where: fedora-host
    provide: koji.nvr:cmake-4.4.2.fc45
  - how: artifact
    where: debian-host
    provide: ???
  - how: install
    package: cmake

then the prepare/install phase there is common for both guests, but it would have different NVRs for each one that it would need to narrow down to.

[thrix]

From triage:

• we are missing an example plan showing the problem, first work on that (2 different distros or 2 different arches)
• needs first designing how it will work, current code is not prepared for this