What needs to get done

Just like we ignore the packed artifacts themselves (like *.snap and *.rock in AppMetadata's source_ignore_patterns), we should also ignore changes in spread/, so that test changes don't cause a needless re-build of the artifact.

This does mean that we're basically prohibiting spread/ from being a part's source.

Why it needs to get done

So that test changes don't cause a needless re-build of the artifact.