The `exe` folder and `lib` folder of default gems don't locate under the
same place. While `exe/irb` will be under the gem folder, `irb.rb` will be
under `lib/ruby/VERSION/`.

So `require_relative` will make `irb` unuseable when shipped with Ruby.

Related discussion in the comments: https://github.com/ruby/irb/pull/335

https://github.com/ruby/irb/commit/99d3aa979d