Add libraries:// stream wrappers to access system files

Postponing this at least to #667058: Add a libraries folder with a README.txt in it to DRUPAL_ROOT. Many open questions regarding libraries in D8.

This only makes sense if we'd have Libraries API in core, which we don't. Something can't be relative to a library, because a library is not an extension. Something can only be relative to extensions, i.e. to modules and themes.