Forbid spurious restriction of a non-public member

I have noticed this in the field:

```java
@Restricted(NoExternalUse.class)
interface Whatever {
}
```

which makes no sense since this type was not public to begin with and so could not have been used externally even without the checker. (Except by something injecting a class into the same package, which we assume is malicious, and solvable anyway via `Sealed: true`.) Since this is a mistake, it should be made into a compile-time error by the annotation processor.