Add names for department to Registry

lib/rubocop/cop/registry.rb

@@ -184,6 +184,10 @@ def names
         cops.map(&:cop_name)
       end
 
+      def names_for_department(department)
+        cops.select { |cop| cop.department == department.to_sym }.map(&:cop_name)
+      end
+
       def ==(other)
         cops == other.cops
       end

spec/rubocop/cop/registry_spec.rb

@@ -350,4 +350,11 @@
       expect(registry.department?('Foo')).to be false
     end
   end
+
+  describe 'names_for_department' do
+    it 'returns array of cops for specified department' do
+      expect(registry.names_for_department('Lint'))
+        .to eq %w[Lint/BooleanSymbol Lint/DuplicateMethods]
+    end
+  end
 end