Fixed #22603 -- Reorganized classes in django.db.backends.

docs/ref/migration-operations.txt

@@ -272,7 +272,7 @@ if supplied) should be callable objects that accept two arguments; the first is
 an instance of ``django.apps.registry.Apps`` containing historical models that
 match the operation's place in the project history, and the second is an
 instance of :class:`SchemaEditor
-<django.db.backends.schema.BaseDatabaseSchemaEditor>`.
+<django.db.backends.base.schema.BaseDatabaseSchemaEditor>`.
 
 The optional ``hints`` argument will be passed as ``**hints`` to the
 :meth:`allow_migrate` method of database routers to assist them in making a

docs/ref/schema-editor.txt

@@ -2,7 +2,7 @@
 ``SchemaEditor``
 ================
 
-.. module:: django.db.backends.schema
+.. module:: django.db.backends.base.schema
 
 .. class:: BaseDatabaseSchemaEditor
 

docs/releases/1.8.txt

@@ -942,6 +942,19 @@ Database backend API
 The following changes to the database backend API are documented to assist
 those writing third-party backends in updating their code:
 
+* ``BaseDatabaseXXX`` classes have been moved to ``django.db.backends.base``.
+  Please import them from the new locations::
+
+    from django.db.backends.base.base import BaseDatabaseWrapper
+    from django.db.backends.base.client import BaseDatabaseClient
+    from django.db.backends.base.creation import BaseDatabaseCreation
+    from django.db.backends.base.features import BaseDatabaseFeatures
+    from django.db.backends.base.introspection import BaseDatabaseIntrospection
+    from django.db.backends.base.introspection import FieldInfo, TableInfo
+    from django.db.backends.base.operations import BaseDatabaseOperations
+    from django.db.backends.base.schema import BaseDatabaseSchemaEditor
+    from django.db.backends.base.validation import BaseDatabaseValidation
+
 * The ``data_types``, ``data_types_suffix``, and
   ``data_type_check_constraints`` attributes have moved from the
   ``DatabaseCreation`` class to ``DatabaseWrapper``.