Adjust semantics of Formula#fetch

It doesn't really make logical sense that this method returns both the
fetched path (or sometimes nil!) and the downloader, so just return the
path (again, or nil!) and callers that want the downloader can ask for
it separately.

Library/Homebrew/cmd/fetch.rb

@@ -25,7 +25,7 @@ module Homebrew extend self
 
   def fetch_formula f
     f.cached_download.rmtree if already_fetched?(f) && ARGV.force?
-    download, _ = f.fetch
+    download = f.fetch
 
     # FIXME why are strategies returning different types?
     return unless download.is_a? Pathname

Library/Homebrew/formula.rb

@@ -618,7 +618,7 @@ class Formula
   def fetch
     # Ensure the cache exists
     HOMEBREW_CACHE.mkpath
-    return downloader.fetch, downloader
+    downloader.fetch
   end
 
   # For FormulaInstaller.
@@ -643,8 +643,8 @@ class Formula
   private
 
   def stage
-    fetched, downloader = fetch
-    verify_download_integrity fetched if fetched.kind_of? Pathname
+    fetched = fetch
+    verify_download_integrity(fetched) if fetched.kind_of? Pathname
     mktemp do
       downloader.stage
       # Set path after the downloader changes the working folder.

Library/Homebrew/formula_installer.rb

@@ -406,8 +406,8 @@ class FormulaInstaller
   end
 
   def pour
-    fetched, downloader = f.fetch
-    f.verify_download_integrity fetched unless downloader.local_bottle_path
+    fetched, downloader = f.fetch, f.downloader
+    f.verify_download_integrity(fetched) unless downloader.local_bottle_path
     HOMEBREW_CELLAR.cd do
       downloader.stage
     end