title: maven plugin - get generated-sources directory
body: <p>I am developing a Maven plugin. Is there a way to get the location of the <code>target\generated-sources</code> directory? It is possible to get the location of the <code>target</code> directory as following:</p>
  
  <pre><code>@Mojo(...)
  public class MyMojo extends AbstractMojo {
      @Parameter(defaultValue = "${project.build.directory}", readonly = true)
      private File target;
  
      ....
  </code></pre>
  
  <p>This gives:</p>
  
  <pre><code>&lt;base dir&gt;\target
  </code></pre>
  
  <p>I need:</p>
  
  <pre><code>&lt;base dir&gt;\target\generated-sources
  </code></pre>
  
  <p>Ofcourse, I can append <code>generated-sources</code> to the path, but it is probably better to rely on the project settings.</p>
  
  <p>So this is not desired:</p>
  
  <pre><code>@Mojo(...)
  public class MyMojo extends AbstractMojo {
      @Parameter(defaultValue = "${project.build.directory}\generated-sources", readonly = true)
      private File target;
  
      ....
  </code></pre>