`AddNullMarkedToPackageInfo` should also report absent packages

[JarvisCraft]

Problem

There is an experimental AddNullMarkedToPackageInfo bug pattern which reports package-info.java files which do not have @NullMarked applied to them.

It does help detect the issue of forgetting to add @NullMarked to the package, but in our experience a more common problem is forgetting to create package-info itself and thus marking it with @NullMarked which is currently not caught by the checker.

Solution

The suggestion is to also handle this scenario, probably also allowing the package to be explicitly @NullUnmarked so that there still is an escape hatch for unannotated packages.

Pseudo-code

Currently:

if (thePackage.exists()) {
    if (!thePackage.isAnnotated(NullMarked.class)) {
        reportError("No @NullMarked on packages %s", thePackage);
    }
}

Suggested:

if (thePackage.exists()) {
    if (!thePackage.isAnnotated(NullMarked.class)
        && !thePackage.isAnnotated(NullUnmarked.class)) {
        reportError("No @NullMarked or @NullUnmarked on packages %s", thePackage);
    }
} else {
    reportError("No package info %s", thePackage);
}