Follow up to #1535

- The removed classes and modules were added back with deprecation
  warning and deprecation test were added for them.
- One test was renamed because it contained `__`.
- Some tests were refactored.
- The ActiveModelSerializers::Deserialization module is now called
  Adapter instead of ActiveModelSerializers::Adapter.
- The changelog was added for #1535

lib/active_model/serializer/adapter.rb

@@ -0,0 +1,49 @@
+module ActiveModel
+  class Serializer
+    # @deprecated Use ActiveModelSerializers::Adapter instead
+    module Adapter
+      class << self
+        def create(resource, options = {})
+          warn_deprecation
+          ActiveModelSerializers::Adapter.create(resource, options)
+        end
+
+        def adapter_class(adapter)
+          warn_deprecation
+          ActiveModelSerializers::Adapter.adapter_class(adapter)
+        end
+
+        def adapter_map
+          warn_deprecation
+          ActiveModelSerializers::Adapter.adapter_map
+        end
+
+        def adapters
+          warn_deprecation
+          ActiveModelSerializers::Adapter.adapters
+        end
+
+        def register(name, klass = name)
+          warn_deprecation
+          ActiveModelSerializers::Adapter.register(name, klass)
+        end
+
+        def lookup(adapter)
+          warn_deprecation
+          ActiveModelSerializers::Adapter.lookup(adapter)
+        end
+
+        def warn_deprecation
+          warn "Calling deprecated #{name} (#{__FILE__}) from #{caller[1..3].join(', ')}. Please use ActiveModelSerializers::Adapter"
+        end
+        private :warn_deprecation
+      end
+
+      require 'active_model/serializer/adapter/base'
+      require 'active_model/serializer/adapter/null'
+      require 'active_model/serializer/adapter/attributes'
+      require 'active_model/serializer/adapter/json'
+      require 'active_model/serializer/adapter/json_api'
+    end
+  end
+end