dependency: TapDependency#tap returns a Tap

Previously, this returned a String, but a Tap instance seems much more
sensible.

I couldn't find anywhere this method was actually used, so the change
shouldn't break anything.

Library/Homebrew/dependency.rb

@@ -175,7 +175,7 @@ class TapDependency < Dependency
   attr_reader :tap
 
   def initialize(name, tags = [], env_proc = DEFAULT_ENV_PROC, option_names = [name.split("/").last])
-    @tap = name.rpartition("/").first
+    @tap = Tap.fetch(name.rpartition("/").first)
     super(name, tags, env_proc, option_names)
   end
 

Library/Homebrew/test/dependency_test.rb

@@ -118,6 +118,11 @@ class DependencyTests < Homebrew::TestCase
 end
 
 class TapDependencyTests < Homebrew::TestCase
+  def test_tap
+    dep = TapDependency.new("foo/bar/dog")
+    assert_equal Tap.new("foo", "bar"), dep.tap
+  end
+
   def test_option_names
     dep = TapDependency.new("foo/bar/dog")
     assert_equal %w[dog], dep.option_names