Improve the Kernel#require patch to avoid warnings

Fix: #461

Also neither Rubygems nor Zeitwerk bother decorating
`Kernel.require`, and no-one requires with `Kernel.require` so
likely not worth it, and if anything that offers an escape hatch to
bypass Bootsnap.

lib/bootsnap/load_path_cache/core_ext/kernel_require.rb

@@ -1,9 +1,9 @@
 # frozen_string_literal: true
 
 module Kernel
-  module_function
+  alias_method :require_without_bootsnap, :require
 
-  alias_method(:require_without_bootsnap, :require)
+  alias_method :require, :require # avoid redefinition warnings
 
   def require(path)
     return require_without_bootsnap(path) unless Bootsnap::LoadPathCache.enabled?
@@ -34,4 +34,6 @@ def require(path)
       return ret
     end
   end
+
+  private :require
 end