title: Turn off "builders" in MapStruct when using Immutables
body: <p>How do I completely disable using "builders" in MapStruct?  I don't want to use them at all as they are causing all kinds of issues for me.</p>
  
  <p>I created the service file under META-INF (I would prefer a way to assign it to the mapping builder= but I did not see any examples how to do it right in code). </p>
  
  <p>It is still trying to use Immutables "builder" instance instance of the "ModifiableXXX" instance I want to map to.  I'd even take a way of forcing it to the modifiable type if that is available.</p>
  
  <p>In another mapping, using an update the ModifiableXXX (with @AfterMapping and @MappingTarget) approach works.</p>
  
  <p>My mapper looks like this right now:</p>
  
  <pre><code>@Mapper
  public interface MongoProjectMapper
  {
      ModifiableProject mapModel(MongoProject project);
  
      @AfterMapping
      ModifiableProject updateProject(MongoEntity e, @MappingTarget ModifiableProject p);
  }
  </code></pre>