Add a new cop to remove require rubygems in gemspecs

There is no reason to do this.

Signed-off-by: Tim Smith <tsmith@chef.io>

config/chefstyle.yml

@@ -642,7 +642,12 @@ ChefRuby/UnlessDefinedRequire:
   Description: Workaround RubyGems' slow requires by only running require if the constant isn't already defined
   Enabled: true
   VersionAdded: '1.3.0'
-  Exclude:
-    - '**/spec/**/*.rb'
-    - '**/omnibus/**/*.rb'
-    - '**/tasks/*.rb'
+  Include:
+    - 'lib/**/*'
+
+ChefRuby/GemspecRequireRubygems:
+  Description: Rubygems does not need to be required in a Gemspec
+  Enabled: true
+  VersionAdded: '1.3.0'
+  Include:
+    - '**/*.gemspec'

lib/rubocop/cop/chef/ruby/gemspec_require_rubygems.rb

@@ -0,0 +1,48 @@
+# frozen_string_literal: true
+#
+# Copyright:: Chef Software, Inc.
+# Author:: Tim Smith (<tsmith@chef.io>)
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+module RuboCop
+  module Cop
+    module Chef
+      module ChefRuby
+        # Rubygems is VERY slow to require gems even if they've already been loaded. To work around this
+        # wrap your require statement with an `if defined?()` check.
+        #
+        class GemspecRequireRubygems < Base
+          extend RuboCop::Cop::AutoCorrector
+          include RangeHelp
+
+          MSG = "Rubygems does not need to be required in a Gemspec"
+
+          def_node_matcher :require_rubygems?, <<-PATTERN
+            (send nil? :require (str "rubygems") )
+          PATTERN
+
+          def on_send(node)
+            require_rubygems?(node) do |r|
+              node = node.parent if node.parent.conditional? # make sure we identify conditionals on the require
+              add_offense(node.loc.expression, message: MSG, severity: :refactor) do |corrector|
+                corrector.remove(range_with_surrounding_space(range: node.loc.expression, side: :left))
+              end
+            end
+          end
+        end
+      end
+    end
+  end
+end