WPCS 2.0: Update the WPCS namespace

[GaryJones]

WordPress/WordPress-Coding-Standards#1555

To prevent conflicts with WordPress itself - especially now that WordPress will let go of the PHP 5.2 minimum supported version -, WPCS 2.0 will not use the WordPress namespace.

While PHPCS 2.x still needed to be supported by WPCS, this could not be helped as it was a PHPCS requirement. Now that PHPCS 2.x support is being dropped in WPCS 2.0, the namespace will be adjusted to be WordPressCS\WordPress.

This is a BC-break for any external standard which extends sniffs from WPCS (like VIPCS currently does, particularly for the Abstract* classes). We will need to update our use statements to use the new WPCS namespace.