in src/main/java/org/apache/maven/plugins/source/AbstractSourceJarMojo.java [366:374]
private Artifact getPreviouslyAttached(Artifact artifact, Project checkProject, String classifier) {
return artifact.getExtension().equals(getExtension())
&& artifact.getGroupId().equals(checkProject.getGroupId())
&& artifact.getArtifactId().equals(checkProject.getArtifactId())
&& artifact.getVersion().toString().equals(checkProject.getVersion())
&& Objects.equals(artifact.getClassifier(), classifier)
? artifact
: null;
}