Allow passing "related_name=None" for autodiscovery (#4810) (#4813)

* Passing "None" as the "related_name" in "Celery.autodiscover_tasks"
  will just try to import the plain package.

* Removes usage of the "imp" module from tests.

* Adds extra tests for the "find_related_module" function.

celery/app/base.py

@@ -655,7 +655,8 @@ class Celery(object):
                 value returned is used (for lazy evaluation).
             related_name (str): The name of the module to find.  Defaults
                 to "tasks": meaning "look for 'module.tasks' for every
-                module in ``packages``."
+                module in ``packages``.".  If ``None`` will only try to import
+                the package, i.e. "look for 'module'".
             force (bool): By default this call is lazy so that the actual
                 auto-discovery won't happen until an application imports
                 the default modules.  Forcing will cause the auto-discovery

celery/loaders/base.py

@@ -253,7 +253,9 @@ def find_related_module(package, related_name):
     # Django 1.7 allows for speciying a class name in INSTALLED_APPS.
     # (Issue #2248).
     try:
-        importlib.import_module(package)
+        module = importlib.import_module(package)
+        if not related_name and module:
+            return module
     except ImportError:
         package, _, _ = package.rpartition('.')
         if not package:

t/unit/app/test_loaders.py

@@ -235,10 +235,17 @@ class test_autodiscovery:
 
     def test_find_related_module(self):
         with patch('importlib.import_module') as imp:
-            with patch('imp.find_module') as find:
-                imp.return_value = Mock()
-                imp.return_value.__path__ = 'foo'
-                base.find_related_module(base, 'tasks')
+            imp.return_value = Mock()
+            imp.return_value.__path__ = 'foo'
+            assert base.find_related_module('bar', 'tasks').__path__ == 'foo'
+            imp.assert_any_call('bar')
+            imp.assert_any_call('bar.tasks')
 
-                find.side_effect = ImportError()
-                base.find_related_module(base, 'tasks')
+            imp.reset_mock()
+            assert base.find_related_module('bar', None).__path__ == 'foo'
+            imp.assert_called_once_with('bar')
+
+            imp.side_effect = ImportError()
+            with pytest.raises(ImportError):
+                base.find_related_module('bar', 'tasks')
+            assert base.find_related_module('bar.foo', 'tasks') is None