Introduce FactoryBot::Internal module

The `FactoryBot` module has a mixture of methods that are meant for use
by people using the library, and methods that are meant only for
internal use. The methods meant for internal use are cluttering the
documentation, and may be confusing to users.

This change was prompted by [#1258]. Rather than introduce yet another
public method on `FactoryBot` meant only for internal use,
we can introduce a `FactoryBot::Internal` module,
and avoid generating documentation for that module.

The `FactoryBot::Internal.register_inline_sequence` method in [#1258]
will need access to the configuration instance, so this PR moves that
into `FactoryBot::Internal`. Eventually I plan to deprecate
`FactoryBot.configuration` and `FactoryBot.reset_configuration`, and to
move more of the `FactoryBot` methods into `FactoryBot::Internal`, but I
would rather hold off on all that until the dust settles on the 5.0
release.

[#1258]: https://github.com/thoughtbot/factory_bot/pull/1258

lib/factory_bot.rb

@@ -45,16 +45,17 @@ require "factory_bot/decorator/invocation_tracker"
 require "factory_bot/decorator/new_constructor"
 require "factory_bot/linter"
 require "factory_bot/version"
+require "factory_bot/internal"
 
 module FactoryBot
   DEPRECATOR = ActiveSupport::Deprecation.new("6.0", "factory_bot")
 
   def self.configuration
-    @configuration ||= Configuration.new
+    Internal.configuration
   end
 
   def self.reset_configuration
-    @configuration = nil
+    Internal.reset_configuration
   end
 
   mattr_accessor :use_parent_strategy, instance_accessor: false

lib/factory_bot/internal.rb

@@ -0,0 +1,14 @@
+module FactoryBot
+  # @api private
+  module Internal
+    class << self
+      def configuration
+        @configuration ||= Configuration.new
+      end
+
+      def reset_configuration
+        @configuration = nil
+      end
+    end
+  end
+end

lib/factory_bot/reload.rb

@@ -1,6 +1,6 @@
 module FactoryBot
   def self.reload
-    reset_configuration
+    Internal.reset_configuration
     register_default_strategies
     register_default_callbacks
     find_definitions

lib/factory_bot/syntax/default.rb

@@ -54,7 +54,7 @@ module FactoryBot
         private
 
         def configuration
-          FactoryBot.configuration
+          FactoryBot::Internal.configuration
         end
       end