Re-raise ModuleNotFoundError unless for guessed task (#8660)

* Modifies integration test to catch actual bad imports.

* Only return none when guessed task-module missing.

* Cleans up test.

* Adds assertion text.

celery/loaders/base.py

@@ -271,7 +271,7 @@ def find_related_module(package, related_name):
     except ModuleNotFoundError as e:
         import_exc_name = getattr(e, 'name', None)
         # If candidate does not exist, then return None.
-        if import_exc_name and module_name.startswith(import_exc_name):
+        if import_exc_name and module_name == import_exc_name:
             return
 
         # Otherwise, raise because error probably originated from a nested import.

t/integration/test_loader.py

@@ -1,3 +1,5 @@
+import pytest
+
 from celery import shared_task
 
 
@@ -7,10 +9,10 @@ def dummy_task(x, y):
 
 
 class test_loader:
-    def test_autodiscovery(self, manager):
+    def test_autodiscovery__when_packages_exist(self, manager):
         # Arrange
         expected_package_name, _, module_name = __name__.rpartition('.')
-        unexpected_package_name = 'nonexistent.package.name'
+        unexpected_package_name = 'datetime.datetime'
 
         # Act
         manager.app.autodiscover_tasks([expected_package_name, unexpected_package_name], module_name, force=True)
@@ -19,4 +21,18 @@ def test_autodiscovery(self, manager):
         assert f'{expected_package_name}.{module_name}.dummy_task' in manager.app.tasks
         assert not any(
             task.startswith(unexpected_package_name) for task in manager.app.tasks
-        )
+        ), 'Expected datetime.datetime to neither have test_loader module nor define a Celery task.'
+
+    def test_autodiscovery__when_packages_do_not_exist(self, manager):
+        # Arrange
+        existent_package_name, _, module_name = __name__.rpartition('.')
+        nonexistent_package_name = 'nonexistent.package.name'
+
+        # Act
+        with pytest.raises(ModuleNotFoundError) as exc:
+            manager.app.autodiscover_tasks(
+                [existent_package_name, nonexistent_package_name], module_name, force=True
+            )
+
+        # Assert
+        assert nonexistent_package_name.startswith(exc.value.name), 'Expected to fail on importing "nonexistent"'