[ruby/irb] Use require_relative to require lib files

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

lib/irb/color.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 require 'reline'
 require 'ripper'
-require 'irb/ruby-lex'
+require_relative 'ruby-lex'
 
 module IRB # :nodoc:
   module Color

lib/irb/color_printer.rb

@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 require 'pp'
-require 'irb/color'
+require_relative 'color'
 
 module IRB
   class ColorPrinter < ::PP

lib/irb/inspector.rb

@@ -114,7 +114,7 @@ module IRB # :nodoc:
     end
     result
   }
-  Inspector.def_inspector([true, :pp, :pretty_inspect], proc{require "irb/color_printer"}){|v|
+  Inspector.def_inspector([true, :pp, :pretty_inspect], proc{require_relative "color_printer"}){|v|
     if IRB.conf[:MAIN_CONTEXT]&.use_colorize?
       IRB::ColorPrinter.pp(v, '').chomp
     else

libexec/irb

@@ -6,6 +6,6 @@
 #   	by Keiju ISHITSUKA(keiju@ruby-lang.org)
 #
 
-require "irb"
+require_relative '../lib/irb'
 
 IRB.start(__FILE__)