require rubygems/package where used

* lib/rubygems/commands/unpack_command.rb: 'rubygems/package' is
  required.  fix the failure of `gem unpack` command.

* lib/rubygems/source/local.rb: ditto.

* lib/rubygems/source/specific_file.rb: ditto.

lib/rubygems/commands/unpack_command.rb

@@ -3,6 +3,7 @@ require 'rubygems/command'
 require 'rubygems/version_option'
 require 'rubygems/security_option'
 require 'rubygems/remote_fetcher'
+require 'rubygems/package'
 
 # forward-declare
 

lib/rubygems/source/local.rb

@@ -3,6 +3,8 @@
 # The local source finds gems in the current directory for fulfilling
 # dependencies.
 
+require 'rubygems/package'
+
 class Gem::Source::Local < Gem::Source
 
   def initialize # :nodoc:

lib/rubygems/source/specific_file.rb

@@ -3,6 +3,8 @@
 # A source representing a single .gem file.  This is used for installation of
 # local gems.
 
+require 'rubygems/package'
+
 class Gem::Source::SpecificFile < Gem::Source
 
   ##