Fixed #31499 -- Stored ModelState.fields into a dict.

This allows the removal of its O(n) .get_field_by_name method and many
other awkward access patterns.

While fields were initially stored in a list to preserve the initial
model definiton field ordering the auto-detector doesn't take field
ordering into account and no operations exists to reorder fields of a
model.

This makes the preservation of the field ordering completely superflous
because field reorganization after the creation of the model state
wouldn't be taken into account.

tests/migrations/test_loader.py

@@ -73,15 +73,12 @@ class LoaderTests(TestCase):
 
         author_state = project_state.models["migrations", "author"]
         self.assertEqual(
-            [x for x, y in author_state.fields],
+            list(author_state.fields),
             ["id", "name", "slug", "age", "rating"]
         )
 
         book_state = project_state.models["migrations", "book"]
-        self.assertEqual(
-            [x for x, y in book_state.fields],
-            ["id", "author"]
-        )
+        self.assertEqual(list(book_state.fields), ['id', 'author'])
 
         # Ensure we've included unmigrated apps in there too
         self.assertIn("basic", project_state.real_apps)
@@ -122,10 +119,7 @@ class LoaderTests(TestCase):
         self.assertEqual(len([m for a, m in project_state.models if a == "migrations"]), 1)
 
         book_state = project_state.models["migrations", "book"]
-        self.assertEqual(
-            [x for x, y in book_state.fields],
-            ["id", "user"]
-        )
+        self.assertEqual(list(book_state.fields), ['id', 'user'])
 
     @override_settings(MIGRATION_MODULES={"migrations": "migrations.test_migrations_run_before"})
     def test_run_before(self):