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/335https://github.com/ruby/irb/commit/99d3aa979d
1. `require` can mislead Ruby to load system irb's files and cause
constant redefined warnings as other code loads the same module/class
from lib folder.
2. Most files already use `require_relative`.
https://github.com/ruby/irb/commit/848d339f2e
