[#1433784] by dww: Added top-level keys for 'project' and 'library'.

When recording and printing the packaging metadata, we want to
separate projects from libraries. This will make it easier for the
packaging script to read this data and do the right thing, and also
prevents clobbering data in case of namespace collisions between
projects and libraries (which can definitely happen).  Therefore,
drupalorg_drush_metadata() now takes a 'type' parameter.