Check that each package-info is annotated with @NullUnmarked or @NullMarked

[rwinch]

It would be nice if a rule that checks that all packages with Java code have a package-info.java file and it includes @NullUnmarked could be added to verify that JSpecify is being applied properly and continues to be applied properly.

[wilkinsona]

Thanks for the suggestion, Rob. This couldn't be enabled all the time as the majority of our code on which Java Format is used is not yet using JSpecify. To deal with this, I wonder if the new check should be opt-in or opt-out. Making it opt-in would reduce friction when upgrading Java Format. Making it opt-out would make it more likely that it catches misconfiguration of JSpecify.