Fixed #35356 -- Deferred self-referential foreign key fields adequately.

While refs #34612 surfaced issues with reverse one-to-one fields
deferrals, it missed that switching to storing remote fields would break
self-referential relationships.

This change switches to storing related objects in the select mask
instead of remote fields to prevent collisions when dealing with
self-referential relationships that might have a different directional
mask.

Despite fixing #21204 introduced a crash under some self-referential
deferral conditions, it was simply not working even before that as it
aggregated the sets of deferred fields by model.

Thanks Joshua van Besouw for the report and Mariusz Felisiak for the
review.

tests/defer_regress/models.py

@@ -10,6 +10,12 @@ class Item(models.Model):
     text = models.TextField(default="xyzzy")
     value = models.IntegerField()
     other_value = models.IntegerField(default=0)
+    source = models.OneToOneField(
+        "self",
+        related_name="destination",
+        on_delete=models.CASCADE,
+        null=True,
+    )
 
 
 class RelatedItem(models.Model):

tests/defer_regress/tests.py

@@ -309,6 +309,27 @@ class DeferRegressionTest(TestCase):
         with self.assertNumQueries(1):
             self.assertEqual(Item.objects.only("request").get(), item)
 
+    def test_self_referential_one_to_one(self):
+        first = Item.objects.create(name="first", value=1)
+        second = Item.objects.create(name="second", value=2, source=first)
+        with self.assertNumQueries(1):
+            deferred_first, deferred_second = (
+                Item.objects.select_related("source", "destination")
+                .only("name", "source__name", "destination__value")
+                .order_by("pk")
+            )
+        with self.assertNumQueries(0):
+            self.assertEqual(deferred_first.name, first.name)
+            self.assertEqual(deferred_second.name, second.name)
+            self.assertEqual(deferred_second.source.name, first.name)
+            self.assertEqual(deferred_first.destination.value, second.value)
+        with self.assertNumQueries(1):
+            self.assertEqual(deferred_first.value, first.value)
+        with self.assertNumQueries(1):
+            self.assertEqual(deferred_second.source.value, first.value)
+        with self.assertNumQueries(1):
+            self.assertEqual(deferred_first.destination.name, second.name)
+
 
 class DeferDeletionSignalsTests(TestCase):
     senders = [Item, Proxy]