Unavailable builder package changed from warning to error

[davidmorgan]

Reported here #4035 (comment)

[davidmorgan]

This is definitely a regression.

That said, I'm not seeing a use case to allow build.yaml files that reference builders in other packages without also having a dependency on that other package ... why not just move the build.yaml to that package?

Let's see how widespread this is before figuring out if it's worth going back to the warning vs keeping it an error.

If this affects you can disable the problem build.yaml by creating one in your own package to override it.

e.g. to disable circle_flags create an empty file called circle_flags.build.yaml in your package.