Fix method redefinition and circular require issues (#1027)

Closes #999

This PR addresses two issues:

Method Redefinition: The user_parameter method was being defined multiple times
in lib/clearance/configuration.rb. This has been fixed by changing attr_accessor
to attr_writer, ensuring the method is defined only once.

Circular Require: Circular dependencies between lib/clearance.rb and
lib/clearance/engine.rb have been resolved by rearranging the require statements
to avoid circular loading.

lib/clearance.rb

@@ -5,9 +5,9 @@
 require 'clearance/back_door'
 require 'clearance/controller'
 require 'clearance/user'
-require 'clearance/engine'
 require 'clearance/password_strategies'
 require 'clearance/constraints'
+require 'clearance/engine'
 
 module Clearance
 end

lib/clearance/configuration.rb

@@ -135,7 +135,7 @@ class Configuration
     # The parameter for user routes. By default this is derived from the user
     # model.
     # @return [Symbol]
-    attr_accessor :user_parameter
+    attr_writer :user_parameter
 
     # Controls wether users are automatically signed in after successfully
     # resetting their password.

lib/clearance/engine.rb

@@ -1,4 +1,3 @@
-require "clearance"
 require "rails/engine"
 
 module Clearance