Allowed custom querysets when prefetching single valued relations

The original patch for custom prefetches didn't allow usage of custom
queryset for single valued relations (along ForeignKey or OneToOneKey).
Allowing these enables calling performance oriented queryset methods like
select_related or defer/only.

Thanks @akaariai and @timgraham for the reviews. Refs #17001.

tests/prefetch_related/models.py

@@ -170,8 +170,10 @@ class Comment(models.Model):
 ## Models for lookup ordering tests
 
 class House(models.Model):
+    name = models.CharField(max_length=50)
     address = models.CharField(max_length=255)
     owner = models.ForeignKey('Person', null=True)
+    main_room = models.OneToOneField('Room', related_name='main_room_of', null=True)
 
     class Meta:
         ordering = ['id']