Remove the direct url writer from Resource

Library/Homebrew/patch.rb

@@ -161,7 +161,7 @@ end
 class LegacyPatch < ExternalPatch
   def initialize(strip, url)
     super(strip)
-    resource.url = url
+    resource.url(url)
     resource.download_strategy = CurlDownloadStrategy
   end
 

Library/Homebrew/resource.rb

@@ -9,7 +9,7 @@ class Resource
   include FileUtils
 
   attr_reader :checksum, :mirrors, :specs, :using
-  attr_writer :url, :checksum, :version
+  attr_writer :checksum, :version
   attr_accessor :download_strategy
 
   # Formula name must be set after the DSL, as we have no access to the

Library/Homebrew/software_spec.rb

@@ -224,7 +224,7 @@ class Bottle
     checksum, tag = spec.checksum_for(bottle_tag)
 
     filename = Filename.create(formula, tag, spec.revision)
-    @resource.url = build_url(spec.root_url, filename)
+    @resource.url(build_url(spec.root_url, filename))
     @resource.download_strategy = CurlBottleDownloadStrategy
     @resource.version = formula.pkg_version
     @resource.checksum = checksum