Fixed NameError for Gem::Ext due to re-entering file lookup in
  RubyGems' overridden require.  Bug by Koichi Sasada.

  Fixed possible circular require warning in tests.

  Used existing constant for `gem install -g` dependecy file list.

* test/rubygems:  ditto.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43300 b2dd03c8-39d4-4d8f-98ff-823fe69b080e