Ref elastic/kibana#151062
Ref elastic/integrations#5267

In the case where a package references a stream that doesn't exist, the linter should error if the {stream}.yml.hbs file doesn't exist. Similarly, if no streams are defined, the linter should error if the stream.yml.hbs default file doesn't exist.

This will prevent invalid packages from making their way to Fleet, though we'll also be adding some validation on Fleet's end to make sure this doesn't cause crashes.