Added bundler as default gems. Revisit [Feature #12733]

* bin/*, lib/bundler/*, lib/bundler.rb, spec/bundler, man/*:
    Merge from latest stable branch of bundler/bundler repository and
    added workaround patches. I will backport them into upstream.
  * common.mk, defs/gmake.mk: Added `test-bundler` task for test suite
    of bundler.
  * tool/sync_default_gems.rb: Added sync task for bundler.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65509 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/bundler/source/gemspec.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+module Bundler
+  class Source
+    class Gemspec < Path
+      attr_reader :gemspec
+
+      def initialize(options)
+        super
+        @gemspec = options["gemspec"]
+      end
+
+      def as_path_source
+        Path.new(options)
+      end
+    end
+  end
+end