add Layer#component_defined? method (#133, rggen/rggen#171)

lib/rggen/core/builder/layer.rb

@@ -50,6 +50,10 @@ def shared_context(&body)
           current_shared_context(false)
         end
 
+        def component_defined?(component_name)
+          @feature_registries.key?(component_name)
+        end
+
         def define_feature(feature_names, &body)
           Array(feature_names).each do |feature_name|
             do_proxy_call(__method__, nil, feature_name, &body)

spec/rggen/core/builder/layer_spec.rb

@@ -22,6 +22,16 @@
     layer.add_feature_registry(:buzz, buzz_feature_registry)
   end
 
+  describe '#component_defined?' do
+    it '指定したコンポーネントが定義済みかどうかを返す' do
+      expect(layer.component_defined?(:fizz)).to eq true
+      expect(layer.component_defined?(:buzz)).to eq true
+      expect(layer.component_defined?(:foo)).to eq false
+      expect(layer.component_defined?(:bar)).to eq false
+
+    end
+  end
+
   describe 'フィーチャーの定義' do
     specify '#add_feature_registry呼び出し時に指定した登録名でフィーチャーを定義できる' do
       expect(fizz_feature_registry).to receive(:define_feature).with(:foo_0).and_call_original