kernel: Introduce the DirPackage packages to help with development

Add a way to load a Package from the package.xml and not go through
the creation of a star file. This can make developing with packages
more effective.

2013-03-24  Holger Hans Peter Freyther  <holger@freyther.de>

	* libgst/files.c: Add DirPackage.st to the bootstrap.
	* kernel/DirPackage.st: Add new file with the DirPackage class and
	extension to the PackageLoader.
	* kernel/PkgLoader.st: Refactor and create ExternalPackage base class.