FindVulkan: Search for VkLayer libraries on iOS

On non-iOS systems, Vulkan Layers are loaded dynamically by the Vulkan
Loader at run-time.  On iOS systems, they are provided as frameworks to
be bundled with an application, so there must be a way to discover those
dependencies as well.

Inspired by the FindVulkan module in `KhronosGroup/Vulkan-Samples`.

Co-Authored-By: Brad King <brad.king@kitware.com>