Warning for package visibility top-level classes that don't match file name

If the top-level class is `public` and doesn't match the file name, an error is given:
```java
//B.java
public class A {} // The public type A must be defined in its own file
```
However, for package access there's no indication of a mismatch (giving an error is wrong), so I suggest a warning along the lines of: "The package-private type A does not have the name of its own file".