rdoc should not depend on gems
It seems in rdoc code that it can run without gems.
However, at some places, it explicitly cites gem or Gem without dealing with
the exception. Also, it requires "rubygems", which might be useless since
gem_prelude does that.
I created a patch to remove the extra 'require "rubygems"' and treat the gem "xxx"
exception just like /usr/bin/rdoc does