Deduplicate requirements

Because of some quirks in how formulae are loaded, DSL methods invoked
in the class definition are called multiple times. This results in, for
example, duplicate dependencies and requirements.

Code that iterates over requirements and takes some action for each can
thus repeat things that shouldn't be repeated, like appending to
environment variables.

Make DependencyCollector's external_deps a Set, and define eql? and hash
on Requirement to prevent these duplicates.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/dependencies.rb

@@ -1,3 +1,5 @@
+require 'set'
+
 ## This file defines dependencies and requirements.
 ##
 ## A dependency is a formula that another formula needs to install.
@@ -21,7 +23,7 @@ class DependencyCollector
 
   def initialize
     @deps = Dependencies.new
-    @external_deps = []
+    @external_deps = Set.new
   end
 
   def add spec
@@ -119,6 +121,14 @@ class Requirement
   def fatal?; false; end
   def message; ""; end
   def modify_build_environment; nil end
+
+  def eql?(other)
+    other.is_a? self.class and hash == other.hash
+  end
+
+  def hash
+    @message.hash
+  end
 end