Require the top file and autoload Base class

The generators entry point file is rails/generators so we need to make
sure uers will be able to use the generators when requiring that file.

Fixes #12802

railties/lib/rails/generators.rb

@@ -15,6 +15,7 @@ module Rails
   module Generators
     autoload :Actions,         'rails/generators/actions'
     autoload :ActiveModel,     'rails/generators/active_model'
+    autoload :Base,            'rails/generators/base'
     autoload :Migration,       'rails/generators/migration'
     autoload :NamedBase,       'rails/generators/named_base'
     autoload :ResourceHelpers, 'rails/generators/resource_helpers'

railties/lib/rails/generators/app_base.rb

@@ -3,7 +3,7 @@ require 'active_support/core_ext/string/strip'
 require 'rails/version' unless defined?(Rails::VERSION)
 require 'open-uri'
 require 'uri'
-require 'rails/generators/base'
+require 'rails/generators'
 require 'active_support/core_ext/array/extract_options'
 
 module Rails

railties/lib/rails/generators/base.rb

@@ -7,8 +7,6 @@ rescue LoadError
   exit
 end
 
-require 'rails/generators'
-
 module Rails
   module Generators
     class Error < Thor::Error # :nodoc: