What needs to get done

Check if the build plan will create an artefact that can only be produced by cross-compiling.

If it will, then raise an error before sending the project to Launchpad.

Additionally, we need to add a task in craft-application to upstream Snapcraft's build-plan validation. This includes this cross-compile check and a check for the same build-for for multiple platforms.

Why it needs to get done

Launchpad does not support cross-compiling or have a concept of build-on vs build-for architectures. This is a problem for Snapcraft, because remote-build accepts a --build-for argument but has to provide the build-on to Launchpad. This is why build-on has to be the same as build-for.

This is a poor UX because the project will be uploaded to Launchpad and Launchpad will not build any artefacts or provide useful error messages.

Example:

platforms:
  platform1:
    build-on: amd64
    build-for: riscv64