Source Xref site page does handle Java module projects properly.

### Affected version

3.6.0

### Bug description

[This project](https://github.com/bradleylarrick/maven-jxr-error.git) demonstrates the problems Maven JXR has with projects that implement Java modules.

The resulting site pages for `Source Xref` include an empty `(default package)` entry, and the Javadoc link within the source code page uses the directory structure of a non-module Java project:
```
file:///C:/projects/maven-jxr-error/target/site/apidocs/org/natuna/jxrerror/NullSafe.html
```
instead of the structure used in Java module projects:
```
file:///C:/projects/maven-jxr-error/target/site/apidocs/org.natuna.jxrerror/org/natuna/jxrerror/NullSafe.html
```

Reproduce the issue with the linked maven-jxr-error project on Maven JXR 3.6.0 and inspect the generated Source Xref pages and Javadoc links for its Java module. Trace the Source Xref generation entry point that produces the empty default-package entry and link, then verify that module-qualified paths are used and the unwanted entry is gone.

Written by the indexing model from the issue text.