title: MEF: How to manually configure Export for a contract implementation
body: <p>I have a MEF's CompositionContainer, a contract (say IFoo) and a module (Prism module but it doesn't matter much here, just some component).
  I want in my module to register the contract's implementation (FooImpl).</p>
  
  <p>If it's Unity I'd do like this:</p>
  
  <pre><code>unity.RegisterType&lt;IFoo, FooImpl&gt;().
  </code></pre>
  
  <p>That's all.
  With MEF I've puzzled. I have to mark my implementation with ExportAttribute, but that lead to it'll be exported automatically. I want to manage this by myself.
  Take a look at the code:</p>
  
  <pre><code>class MyModule: IModule {
    private CompositionContainer m_container;
    public MyModule(CompositionContainer container) {
      m_container = container;
    }
    public void Initialize() {
      ??? I want something like m_container.CreateExport&lt;IFoo, FooImpl&gt;()
    }
  }
  
  public interface IFoo {}
  public class FooImpl : IFoo {
      //[ImportingConstructor]
      public FooImpl(ISomeService svc) {}
  }
  </code></pre>
  
  <p>In Initialize I want manualy <em>export</em> FooImpl as IFoo contract not relying on ExportAttribute on FooImpl class. 
  I understand that I just can create an instance FooImpl (in MyModule.Initialize above), but 
  FooImpl has constructor dependencies on other component/services and I want they to be resolved on creation.</p>
  
  <p>So probably I should ask: how to manually add export having a CompositionContainer instance and a contract? And then mark this export somehow as it has ImportingConstructorAttribute?</p>