layers: simplify v0 function implementations

Since each of the layer library contains only a layer, we can simplify v0
functions by making them wrappers to the contained layer's corresponding
commands.

While we have to make changes to each layer individually, all the changes
should be the same in spirit.  The exception is vk-layer-generate.py where
the missing vkEnumerateDeviceExtensionProperties is also added.
7 files changed