AggregatorSourceJarMojo silently does nothing for non-POM packaging

`AggregatorSourceJarMojo.java:35-37`:

```java
@Override
protected void doExecute() throws MojoException {
if (Type.POM.equals(getProject().getPackaging().type().id())) {
packageSources(reactorProjects);
}
}
```

If the aggregator project has non-POM packaging, the method returns without any warning or error message. While `@Mojo(aggregator = true)` typically implies POM packaging, a misleading build or configuration could lead to silent failure — no source JAR is produced and the user receives no indication why.

**Fix**: Add an `else` branch with a warning or info message explaining why no sources were packaged.

Start in AggregatorSourceJarMojo.java at doExecute(), especially lines 35-37, and inspect how nearby Maven plugin code reports skipped work. Add the requested warning or info message for non-POM packaging, then verify that the POM path still packages sources and the non-POM path explains why no source JAR was produced.

Written by the indexing model from the issue text.