Hide the downloader implementation from the installer

Library/Homebrew/formula_installer.rb

@@ -634,9 +634,8 @@ class FormulaInstaller
     if f.local_bottle_path
       downloader = LocalBottleDownloadStrategy.new(f)
     else
-      bottle = f.bottle
-      downloader = bottle.downloader
-      bottle.verify_download_integrity(bottle.fetch)
+      downloader = f.bottle
+      downloader.verify_download_integrity(downloader.fetch)
     end
     HOMEBREW_CELLAR.cd do
       downloader.stage

Library/Homebrew/software_spec.rb

@@ -170,6 +170,10 @@ class Bottle
     cellar == :any || cellar == HOMEBREW_CELLAR.to_s
   end
 
+  def stage
+    resource.downloader.stage
+  end
+
   private
 
   def build_url(root_url, filename)