Refactored listing template subdirectories in apps.

This change has the nice side effect of removing code that ran at import
time and depended on the app registry at module level -- a notorious
cause of AppRegistryNotReady exceptions.

django/template/utils.py

@@ -0,0 +1,29 @@
+import os
+import sys
+
+from django.apps import apps
+from django.utils import lru_cache
+from django.utils import six
+
+
+@lru_cache.lru_cache()
+def get_app_template_dirs(dirname):
+    """
+    Return an iterable of paths of directories to load app templates from.
+
+    dirname is the name of the subdirectory containing templates inside
+    installed applications.
+    """
+    if six.PY2:
+        fs_encoding = sys.getfilesystemencoding() or sys.getdefaultencoding()
+    template_dirs = []
+    for app_config in apps.get_app_configs():
+        if not app_config.path:
+            continue
+        template_dir = os.path.join(app_config.path, dirname)
+        if os.path.isdir(template_dir):
+            if six.PY2:
+                template_dir = template_dir.decode(fs_encoding)
+            template_dirs.append(template_dir)
+    # Immutable return value because it will be cached and shared by callers.
+    return tuple(template_dirs)