Add support for XP6

This PR adds support for XP6; this includes:

* building releases of xp-framework:core
* building releases of apps depending on xp-framework:core where version >= 6.0

In XP 6, the changes are:
* the tools/` directory does not longer contain `xar.php`
* `class.php` is now `class-main.php`, `web.php` is now `web-main.php`.

The plugin will transfer any files in the `tools/` directory of the core artifact.