Warn when the aggregate goal skips a non-pom project

[kratos0718]

Fixes #306.

Problem

AggregatorSourceJarMojo.doExecute() packages sources only when the project uses pom packaging:

if (Type.POM.equals(getProject().getPackaging().type().id())) {
    packageSources(reactorProjects);
}

With any other packaging it returns silently. No source JAR is produced and the user gets no indication why — which, as the issue notes, is a plausible outcome of a misleading build or configuration even though @Mojo(aggregator = true) normally implies pom.

Change

Adds an else branch that logs a warning naming the goal, the required packaging, and the project's actual packaging:

[WARNING] Not packaging aggregated sources: the aggregate goal requires "pom"
packaging, but this project uses "jar" packaging.

The goal name is now a constant shared by the @Mojo annotation and the message, so the two cannot drift apart.

On warning vs failing

@elharo asked in the issue whether this should fail rather than log. I went with a warning, for the reason discussed there: a non-pom aggregator run succeeds today, so failing would turn currently-green builds red without any change on the user's side, and the people most likely to hit it are exactly those with an incidental binding they never noticed. The complaint in the issue is diagnosability, which the warning resolves fully.

If the project would rather treat it as an outright misconfiguration, I am happy to switch this to a failure — I would just suggest tying that to a major version. Say the word and I will push the change.

Tests

AggregatorSourceJarMojoTest adds two cases:

• warnsWhenPackagingIsNotPom — asserts the warning is emitted and contains the goal, the required packaging and the actual packaging
• doesNotWarnWhenPackagingIsPom — asserts no warning on a legitimate pom run, so the change cannot over-correct into warning on valid builds

Tests run: 2, Failures: 0, Errors: 0, Skipped: 0

Behaviour for pom packaging is unchanged.