PublishPress Checklists Pro v.2.9.0

* Changed: Updated internal libraries to latest versions;
* Changed: Move dependencies to lib/vendor;
* Changed: Internal dependencies moved from `vendor` to `lib/vendor`;
* Changed: Updated internal libraries to the latest versions;
* Changed: Removed the `vendor-locator-checklists` library. Internal vendor is now on a fixed path, `lib/vendor`;
* Changed: Deprecated constant `PUBLISHPRESS_CHECKLISTS_PRO_VENDOR_PATH` in favor of `PPCHPRO_LIB_VENDOR_PATH`;
* Fixed: Fix compatibility with Composer-based installations, using prefixed libraries;