Plugins can't get project properties

For a plugin to get the project's properties, it should define a field as follows:

    /**
     * The whole project.
     * @parameter expression="${project.properties}"
     * @required
     * @readonly
     */
    private java.util.Properties properties;

However, because of a bug in Plexus (see PLX-327), these properties are always empty. This Plexus bug needs to be fixed and Maven should include the fixed plexus-container-default library.