Defer several build libraries loading until needed

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