title: Rename nested annotated field via Django ORM
body: <p>Is it possible to renaming nested fields using for group by clause?</p>
  
  <p>This query: </p>
  
  <pre><code>paymentitem.objects.filter(payment=p).values('item__vat_tax').annotate(base=models.Sum('price')).order_by('item__vat_tax')
  </code></pre>
  
  <p>returns expected data:</p>
  
  <pre><code>&lt;QuerySet [{'base': Decimal('41.322'), 'item__vat_tax': 15}, {'base': Dec
      imal('483.470'), 'item__vat_tax': 21}]&gt;
  </code></pre>
  
  <p>I want to rename field '<em>item__vat_tax</em>' as '<em>vat</em>'. This query:</p>
  
  <pre><code>paymentitem.objects.filter(payment=p).extra(select={'vat': 'item__vat_tax'}).values('item__vat_tax').annotate(base=models.Sum('price')).order_by('vat')
  </code></pre>
  
  <p>return the same result but surprisingly ordered by <em>vat</em> too. </p>
  
  <p>If I change the field name inside the value statement, it raise an error. </p>