Fixed #17485 -- Made defer work with select_related

This commit tackles a couple of issues. First, in certain cases there
were some mixups if field.attname or field.name should be deferred.
Field.attname is now always used.

Another issue tackled is a case where field is both deferred by
.only(), and selected by select_related. This case is now an error.

A lot of thanks to koniiiik (Michal Petrucha) for the patch, and
to Andrei Antoukh for review.

django/db/models/sql/query.py

@@ -1845,9 +1845,15 @@ class Query(object):
 
         If no fields are marked for deferral, returns an empty dictionary.
         """
-        collection = {}
-        self.deferred_to_data(collection, self.get_loaded_field_names_cb)
-        return collection
+        # We cache this because we call this function multiple times
+        # (compiler.fill_related_selections, query.iterator)
+        try:
+            return self._loaded_field_names_cache
+        except AttributeError:
+            collection = {}
+            self.deferred_to_data(collection, self.get_loaded_field_names_cb)
+            self._loaded_field_names_cache = collection
+            return collection
 
     def get_loaded_field_names_cb(self, target, model, fields):
         """