List of derivatives.
Associative array keyed by 'provider__layoutname' where provider is the module or theme name and layoutname is the .yml filename, such as 'bartik__page' or 'layout__one-col'. The values of the array are associative arrays themselves with metadata about the layout such as 'template', 'css', 'admin css' and so on.
Type: array
protected $derivatives = array();