Delay requiring irb until runtime

Fixes Homebrew/homebrew#32410.

Library/Homebrew/debrew.rb

@@ -1,5 +1,4 @@
 require "mutex_m"
-require "debrew/irb" unless ENV["HOMEBREW_NO_READLINE"]
 
 module Debrew
   extend Mutex_m
@@ -130,6 +129,9 @@ module Debrew
           menu.choice(:ignore) { return :ignore } if Ignorable === e
           menu.choice(:backtrace) { puts e.backtrace }
 
+          unless ENV["HOMEBREW_NO_READLINE"]
+            require "debrew/irb"
+
             menu.choice(:irb) do
               puts "When you exit this IRB session, execution will continue."
               set_trace_func proc { |event, _, _, id, binding, klass|
@@ -140,7 +142,8 @@ module Debrew
               }
 
               return :ignore
-          end if Object.const_defined?(:IRB) && Ignorable === e
+            end if Ignorable === e
+          end
 
           menu.choice(:shell) do
             puts "When you exit this shell, you will return to the menu."