Warn when aggregate runs on non-POM packaging

[kratos0718]

Fixes #306

Problem

AggregatorSourceJarMojo.doExecute() packages sources only for pom packaging:

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

Any other packaging falls off the end of the method. No archive is produced and nothing is logged, so there is no way to tell the skip apart from a build that simply did not reach the goal.

Fix

Add the else branch and warn, naming the project and its actual packaging so the message says what to change. The wording follows the classifier warning packageSources already emits.

Testing

AggregatorSourceJarMojoTest covers both branches, overriding packageSources to record the projects it receives rather than building an archive:

• pom packaging still packages the reactor projects, and logs nothing
• non-POM packaging packages nothing and warns, with the message naming the packaging and the project

The second fails against the silent version with Wanted but not invoked; the first passes either way, since that branch is unchanged. mvn verify -DskipITs: 12 tests, all passing.

• I hereby declare this contribution to be licenced under the Apache License Version 2.0, January 2004