`DefaultPackage` emitted for compact source files.

The `DefaultPackage` warning gets emitted for compact source files.

```
/my/project/app/src/main/java/Main.java:1: warning: [DefaultPackage] Java classes shouldn't use default package
public static final void main() {}
^
(see https://errorprone.info/bugpattern/DefaultPackage)
```

This feels wrong to me since compact source files by design put their implicitly defined class in the unnamed (default) package. It the intention is to dissuade the use of compact source files as well, then I think it should be part of a different warning.