Provide control over precedence of org.apache.maven.plugins group in search path

settings.xml has a <pluginGroups> section to list additional groupIds to search for top level m2 plugin goals.

By default, "org.apache.maven.plugins" is not required to be present in this list, and always wins if there is a collision.

<pluginGroups> that explicitly list "org.apache.maven.plugins" should be able to control this default behavior, so that a different plugin groupId can always win in the case of a collision.

For example, in settings.xml:

<pluginGroups>
<pluginGroup>org.example.maven.plugins</pluginGroup>
<pluginGroup>org.apache.maven.plugins</pluginGroup>
</pluginGroups>

This would allow "org.example.maven.plugins" to always win in the case of a collision, whereas:

<pluginGroups>
<pluginGroup>org.example.maven.plugins</pluginGroup>
</pluginGroups>

would still allow "org.apache.maven.plugins" to always win, as it does today.