[1.8.x] Fixed #24848 -- Fixed ValueError for faulty migrations module.

Added apps to unmigrated apps if the migrations module is a file
or a folder missing __init__.py.

Thanks to Ernest0x for the bug report.

Backport of d73176a842 from master

django/db/migrations/loader.py

@@ -77,9 +77,11 @@ class MigrationLoader(object):
             else:
                 # PY3 will happily import empty dirs as namespaces.
                 if not hasattr(module, '__file__'):
+                    self.unmigrated_apps.add(app_config.label)
                     continue
                 # Module is not a package (e.g. migrations.py).
                 if not hasattr(module, '__path__'):
+                    self.unmigrated_apps.add(app_config.label)
                     continue
                 # Force a reload if it's already loaded (tests need this)
                 if was_loaded:

docs/releases/1.8.3.txt

@@ -11,3 +11,6 @@ Bugfixes
 
 * Fixed ``BaseRangeField.prepare_value()`` to use each ``base_field``’s
   ``prepare_value()`` method (:ticket:`24841`).
+
+* Fixed crash during :djadmin:`makemigrations` if a migrations module either
+  is missing ``__init__.py`` or is a file (:ticket:`24848`).

tests/migrations/test_loader.py

@@ -164,12 +164,20 @@ class LoaderTests(TestCase):
 
     def test_load_module_file(self):
         with override_settings(MIGRATION_MODULES={"migrations": "migrations.faulty_migrations.file"}):
-            MigrationLoader(connection)
+            loader = MigrationLoader(connection)
+            self.assertIn(
+                "migrations", loader.unmigrated_apps,
+                "App with migrations module file not in unmigrated apps."
+            )
 
     @skipIf(six.PY2, "PY2 doesn't load empty dirs.")
     def test_load_empty_dir(self):
         with override_settings(MIGRATION_MODULES={"migrations": "migrations.faulty_migrations.namespace"}):
-            MigrationLoader(connection)
+            loader = MigrationLoader(connection)
+            self.assertIn(
+                "migrations", loader.unmigrated_apps,
+                "App missing __init__.py in migrations module not in unmigrated apps."
+            )
 
     @override_settings(MIGRATION_MODULES={"migrations": "migrations.test_migrations_squashed"})
     def test_loading_squashed(self):