Defer several build libraries loading until needed

Rakefile

@@ -10,8 +10,6 @@ $LOAD_PATH << File.join(MRUBY_ROOT, "lib")
 # load build systems
 require "mruby-core-ext"
 require "mruby/build"
-require "mruby/gem"
-require "mruby/lockfile"
 
 # load configuration file
 MRUBY_CONFIG = (ENV['MRUBY_CONFIG'] && ENV['MRUBY_CONFIG'] != '') ? ENV['MRUBY_CONFIG'] : "#{MRUBY_ROOT}/build_config.rb"

lib/mruby-core-ext.rb

@@ -1,3 +1,5 @@
+autoload :Pathname, 'pathname'
+
 class Object
   class << self
     def attr_block(*syms)

lib/mruby/build.rb

@@ -1,7 +1,11 @@
+require "mruby-core-ext"
 require "mruby/build/load_gems"
 require "mruby/build/command"
 
 module MRuby
+  autoload :Gem, "mruby/gem"
+  autoload :Lockfile, "mruby/lockfile"
+
   class << self
     def targets
       @targets ||= {}

lib/mruby/gem.rb

@@ -1,7 +1,6 @@
-require 'pathname'
 require 'forwardable'
-require 'tsort'
-require 'shellwords'
+autoload :TSort, 'tsort'
+autoload :Shellwords, 'shellwords'
 
 module MRuby
   module Gem