[regression] plugin updates are requested on every build regardless of policies

I'm not sure if this is an intentional change that I've missed or not, but running something like:

mvn archetype:generate

will retrieve the metadata from the repository on every build:

Downloading: http://repo1.maven.org/maven2/org/apache/maven/plugins/maven-metadata.xml
9 KB downloaded at 17.1 KB/sec

This is in contrast to Maven 2.2.1 which always continues to use the first version encountered until you use -U:

  <pluginRepositories>
    <pluginRepository>
      <releases>
        <updatePolicy>never</updatePolicy>
      </releases>

While that remains in the super POM of Maven 3, it seems that the metadata is retrieved regardless.