Simplified django.db.utils.load_backend().

Previously load_backend() performed search by computing the (sole)
entry of django.db.backends.__path__ manually from
django.db.utils.__file__. Now django.db.backends.__path__ is used
directly.

django/db/utils.py

@@ -1,6 +1,5 @@
 import pkgutil
 from importlib import import_module
-from pathlib import Path
 
 from django.conf import settings
 from django.core.exceptions import ImproperlyConfigured
@@ -113,9 +112,9 @@ def load_backend(backend_name):
     except ImportError as e_user:
         # The database backend wasn't found. Display a helpful error message
         # listing all built-in database backends.
-        backend_dir = str(Path(__file__).parent / 'backends')
+        import django.db.backends
         builtin_backends = [
-            name for _, name, ispkg in pkgutil.iter_modules([backend_dir])
+            name for _, name, ispkg in pkgutil.iter_modules(django.db.backends.__path__)
             if ispkg and name not in {'base', 'dummy', 'postgresql_psycopg2'}
         ]
         if backend_name not in ['django.db.backends.%s' % b for b in builtin_backends]: