Fix some #eql? correctness issues

The implementation of #eql? and #hash should ensure that if a.eql?(b),
then a.hash == b.hash, but #eql? itself should not *depend* on #hash.

For example, given

  class Thingy
    def eql?
      instance_of?(other.class) && hash == other.hash
    end

    def hash
      [name, *tags].hash
    end
  end

if #hash produces a collision for different values of [name, *tags], two
Thingy objects will appear to be eql?, even though this is not the case.
Instead, #eql? should depend on the equality of name and tags directly.

Library/Homebrew/dependency.rb

@@ -21,7 +21,7 @@ class Dependency
   end
 
   def eql?(other)
-    instance_of?(other.class) && hash == other.hash
+    instance_of?(other.class) && name == other.name
   end
 
   def hash

Library/Homebrew/options.rb

@@ -20,7 +20,7 @@ class Option
   end
 
   def eql?(other)
-    other.is_a?(self.class) && hash == other.hash
+    instance_of?(other.class) && name == other.name
   end
 
   def hash

Library/Homebrew/requirement.rb

@@ -54,7 +54,7 @@ class Requirement
   end
 
   def eql?(other)
-    instance_of?(other.class) && hash == other.hash
+    instance_of?(other.class) && name == other.name && tags == other.tags
   end
 
   def hash

Library/Homebrew/test/test_requirement.rb

@@ -121,4 +121,18 @@ class RequirementTests < Test::Unit::TestCase
       req.to_dependency.modify_build_environment
     end
   end
+
+  def test_eql
+    a, b = Requirement.new, Requirement.new
+    assert a.eql?(b)
+    assert b.eql?(a)
+    assert_equal a.hash, b.hash
+  end
+
+  def test_not_eql
+    a, b = Requirement.new([:optional]), Requirement.new
+    assert_not_equal a.hash, b.hash
+    assert !a.eql?(b)
+    assert !b.eql?(a)
+  end
 end