Register autload for Deduplicable

This module is used in many classes but only required in one of them.
Depending in the load order, it could be undefined before its first use.

Instead of adding requires to every single files that use it I
registered an autoload entry for it.

activerecord/lib/active_record/connection_adapters.rb

@@ -13,6 +13,7 @@ module ActiveRecord
     autoload :PoolManager
     autoload :LegacyPoolManager
     autoload :SchemaCache
+    autoload :Deduplicable
 
     autoload_at "active_record/connection_adapters/abstract/schema_definitions" do
       autoload :IndexDefinition

activerecord/lib/active_record/connection_adapters/sql_type_metadata.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-require "active_record/connection_adapters/deduplicable"
-
 module ActiveRecord
   # :stopdoc:
   module ConnectionAdapters