@Component(role=ArtifactMetadataSource.class)
public class DefaultMetadataSource
extends MavenMetadataSource

This realizes the metadata source via the default hint to provide backward-compat with Maven 2.x whose Plexus version registered component descriptors twice: once keyed by role+roleHint and once keyed by role only. This effectively made the metadata source available with its original role hint ("maven") as well as the default hint.