scripts: PV codegen handle multiple requires
scripts: PV codegen handle multiple requires

Modify parameter validation codegen to handle multiple require blocks in
an extension, each with additional extension dependencies.

Change-Id: I336569ca19808f3dca342eb65fd076599c4fb329

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools