Refactor and document Cleanup.

Library/Homebrew/cleanup.rb

@@ -6,8 +6,15 @@ require "formula"
 require "cask/cask_loader"
 require "set"
 
+module Homebrew
+  # Helper class for cleaning up the Homebrew cache.
+  #
+  # @api private
+  class Cleanup
     CLEANUP_DEFAULT_DAYS = 30
+    private_constant :CLEANUP_DEFAULT_DAYS
 
+    # `Pathname` refinement with helper functions for cleaning up files.
     module CleanupRefinement
       refine Pathname do
         def incomplete?
@@ -124,8 +131,6 @@ end
 
     using CleanupRefinement
 
-module Homebrew
-  class Cleanup
     extend Predicable
 
     PERIODIC_CLEAN_FILE = (HOMEBREW_CACHE/".cleaned").freeze

Library/Homebrew/test/cleanup_spec.rb

@@ -5,9 +5,9 @@ require "cleanup"
 require "cask/cache"
 require "fileutils"
 
-using CleanupRefinement
+using Homebrew::Cleanup::CleanupRefinement
 
-describe CleanupRefinement do
+describe Homebrew::Cleanup::CleanupRefinement do
   describe "::prune?" do
     alias_matcher :be_pruned, :be_prune