Suppress RuboCop's offense

This commit suppresses the following offenses:

```console
$ bundle exec rubocop
(snip)

lib/rubocop/cop/mixin/index_method.rb:107:7: W: Lint/UselessConstantScoping: Useless private access modifier for constant scope.
      Captures = ::Struct.new( ...
      ^^^^^^^^^^^^^^^^^^^^^^^^
lib/rubocop/cop/mixin/index_method.rb:117:7: W: Lint/UselessConstantScoping: Useless private access modifier for constant scope.
      Autocorrection = ::Struct.new(:match, :block_node, :leading, :trailing) do ...
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

302 files inspected, 2 offenses detected
```

Follow-up to rubocop/rubocop#13831.

lib/rubocop/cop/mixin/index_method.rb

@@ -6,6 +6,71 @@ module Cop
     module IndexMethod # rubocop:disable Metrics/ModuleLength
       RESTRICT_ON_SEND = %i[each_with_object to_h map collect []].freeze
 
+      # Internal helper class to hold match data
+      Captures = ::Struct.new(
+        :transformed_argname,
+        :transforming_body_expr
+      ) do
+        def noop_transformation?
+          transforming_body_expr.lvar_type? && transforming_body_expr.children == [transformed_argname]
+        end
+      end
+
+      # Internal helper class to hold autocorrect data
+      Autocorrection = ::Struct.new(:match, :block_node, :leading, :trailing) do
+        def self.from_each_with_object(node, match)
+          new(match, node, 0, 0)
+        end
+
+        def self.from_to_h(node, match)
+          new(match, node, 0, 0)
+        end
+
+        def self.from_map_to_h(node, match)
+          if node.block_literal?
+            strip_trailing_chars = 0
+          else
+            map_range = node.children.first.source_range
+            node_range = node.source_range
+            strip_trailing_chars = node_range.end_pos - map_range.end_pos
+          end
+
+          new(match, node.children.first, 0, strip_trailing_chars)
+        end
+
+        def self.from_hash_brackets_map(node, match)
+          new(match, node.children.last, "#{node.receiver.source}[".length, ']'.length)
+        end
+
+        def strip_prefix_and_suffix(node, corrector)
+          expression = node.source_range
+          corrector.remove_leading(expression, leading)
+          corrector.remove_trailing(expression, trailing)
+        end
+
+        def set_new_method_name(new_method_name, corrector)
+          range = block_node.send_node.loc.selector
+          if (send_end = block_node.send_node.loc.end)
+            # If there are arguments (only true in the `each_with_object` case)
+            range = range.begin.join(send_end)
+          end
+          corrector.replace(range, new_method_name)
+        end
+
+        def set_new_arg_name(transformed_argname, corrector)
+          return if block_node.numblock_type?
+
+          corrector.replace(block_node.arguments, "|#{transformed_argname}|")
+        end
+
+        def set_new_body_expression(transforming_body_expr, corrector)
+          body_source = transforming_body_expr.source
+          body_source = "{ #{body_source} }" if transforming_body_expr.hash_type? && !transforming_body_expr.braces?
+
+          corrector.replace(block_node.body, body_source)
+        end
+      end
+
       def on_block(node)
         on_bad_each_with_object(node) do |*match|
           handle_possible_offense(node, match, 'each_with_object')
@@ -102,71 +167,6 @@ def execute_correction(corrector, node, correction)
         correction.set_new_arg_name(captures.transformed_argname, corrector)
         correction.set_new_body_expression(captures.transforming_body_expr, corrector)
       end
-
-      # Internal helper class to hold match data
-      Captures = ::Struct.new(
-        :transformed_argname,
-        :transforming_body_expr
-      ) do
-        def noop_transformation?
-          transforming_body_expr.lvar_type? && transforming_body_expr.children == [transformed_argname]
-        end
-      end
-
-      # Internal helper class to hold autocorrect data
-      Autocorrection = ::Struct.new(:match, :block_node, :leading, :trailing) do
-        def self.from_each_with_object(node, match)
-          new(match, node, 0, 0)
-        end
-
-        def self.from_to_h(node, match)
-          new(match, node, 0, 0)
-        end
-
-        def self.from_map_to_h(node, match)
-          if node.block_literal?
-            strip_trailing_chars = 0
-          else
-            map_range = node.children.first.source_range
-            node_range = node.source_range
-            strip_trailing_chars = node_range.end_pos - map_range.end_pos
-          end
-
-          new(match, node.children.first, 0, strip_trailing_chars)
-        end
-
-        def self.from_hash_brackets_map(node, match)
-          new(match, node.children.last, "#{node.receiver.source}[".length, ']'.length)
-        end
-
-        def strip_prefix_and_suffix(node, corrector)
-          expression = node.source_range
-          corrector.remove_leading(expression, leading)
-          corrector.remove_trailing(expression, trailing)
-        end
-
-        def set_new_method_name(new_method_name, corrector)
-          range = block_node.send_node.loc.selector
-          if (send_end = block_node.send_node.loc.end)
-            # If there are arguments (only true in the `each_with_object` case)
-            range = range.begin.join(send_end)
-          end
-          corrector.replace(range, new_method_name)
-        end
-
-        def set_new_arg_name(transformed_argname, corrector)
-          return if block_node.numblock_type?
-
-          corrector.replace(block_node.arguments, "|#{transformed_argname}|")
-        end
-
-        def set_new_body_expression(transforming_body_expr, corrector)
-          body_source = transforming_body_expr.source
-          body_source = "{ #{body_source} }" if transforming_body_expr.hash_type? && !transforming_body_expr.braces?
-
-          corrector.replace(block_node.body, body_source)
-        end
-      end
     end
   end
 end