[rubygems/rubygems] Avoid standard requires while loading shared helpers

We should make sure Bundler does not trigger RubyGems require logic for
gem activation until it had the chance to register its own monkeypatches
to RubyGems.

https://github.com/rubygems/rubygems/commit/fbd2ff86b9

lib/bundler/constants.rb

@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require "rbconfig"
+
 module Bundler
   WINDOWS = RbConfig::CONFIG["host_os"] =~ /(msdos|mswin|djgpp|mingw)/
   FREEBSD = RbConfig::CONFIG["host_os"].to_s.include?("bsd")

lib/bundler/rubygems_integration.rb

@@ -4,9 +4,7 @@ require "rubygems" unless defined?(Gem)
 
 module Bundler
   class RubygemsIntegration
-    require "monitor"
-
-    EXT_LOCK = Monitor.new
+    autoload :Monitor, "monitor"
 
     def initialize
       @replaced_methods = {}
@@ -173,7 +171,7 @@ module Bundler
     end
 
     def ext_lock
-      EXT_LOCK
+      @ext_lock ||= Monitor.new
     end
 
     def spec_from_gem(path)

lib/bundler/shared_helpers.rb

@@ -1,15 +1,17 @@
 # frozen_string_literal: true
 
-require "pathname"
-require "rbconfig"
-
 require_relative "version"
-require_relative "constants"
 require_relative "rubygems_integration"
 require_relative "current_ruby"
 
 module Bundler
+  autoload :WINDOWS, File.expand_path("constants", __dir__)
+  autoload :FREEBSD, File.expand_path("constants", __dir__)
+  autoload :NULL, File.expand_path("constants", __dir__)
+
   module SharedHelpers
+    autoload :Pathname, "pathname"
+
     def root
       gemfile = find_gemfile
       raise GemfileNotFound, "Could not locate Gemfile" unless gemfile