Pass the downloader a proxy object instead of the resource

We need to limit the interface that is exposed to the downloader in
order to make future changes easier.

This will be important for work on new features, such as selecting
a mirror from the command line.

In the future, the existing Resource class will probably be split into
multiple classes.

Library/Homebrew/resource.rb

@@ -16,6 +16,28 @@ class Resource
   # formula name before initialization of the formula
   attr_accessor :name, :owner
 
+  class Download
+    def initialize(resource)
+      @resource = resource
+    end
+
+    def url
+      @resource.url
+    end
+
+    def specs
+      @resource.specs
+    end
+
+    def version
+      @resource.version
+    end
+
+    def mirrors
+      @resource.mirrors
+    end
+  end
+
   def initialize name=nil, &block
     @name = name
     @url = nil
@@ -28,7 +50,7 @@ class Resource
   end
 
   def downloader
-    @downloader ||= download_strategy.new(download_name, self)
+    @downloader ||= download_strategy.new(download_name, Download.new(self))
   end
 
   # Removes /s from resource names; this allows go package names