Ensure :cairo and :pixman resolve to standard formula deps

We do not use X11 to satisfy these deps for consistency reasons,
but we should continue to support the symbols for compatibility.

Library/Homebrew/dependency_collector.rb

@@ -83,6 +83,10 @@ private
       else
         X11Dependency::Proxy.for(spec.to_s, tag)
       end
+    when :cairo, :pixman
+      # We no longer use X11 psuedo-deps for cairo or pixman,
+      # so just return a standard formula dependency.
+      Dependency.new(spec.to_s, tag)
     when :x11        then X11Dependency.new(spec.to_s, tag)
     when :xcode      then XcodeDependency.new(tag)
     when :mysql      then MysqlInstalled.new(tag)

Library/Homebrew/requirements.rb

@@ -107,7 +107,7 @@ class X11Dependency < Requirement
   # Rather than instantiate it directly, a separate class is built
   # for each of the packages that we proxy to X11Dependency.
   class Proxy < self
-    PACKAGES = [:libpng, :freetype, :pixman, :fontconfig]
+    PACKAGES = [:libpng, :freetype, :fontconfig]
 
     def self.for(name, *tags)
       constant = name.capitalize