VK_LAYER_PATH equivalent for implicit layers

[baldurk]

What enhancement are you suggesting for the Vulkan Loader? Please describe in detail. I built a local copy of Fossilize and I wanted to point at it temporarily for an application run, but VK_LAYER_PATH and VK_ADD_LAYER_PATH didn't function as I expected. It took a while and digging through the source to realise that they only apply to explicit layers - though I went back to the docs and noticed that it does indeed say "when looking for explicit layer manifest files.".

I'm not sure the reasoning for this exactly, but not all layers are natively explicit layers so it would be nice to be able to extend the search paths similarly for implicit layers. I don't think this is a security consideration since I was able to just force the layer as if it were explicit via VK_INSTANCE_LAYERS once I twigged.

As a side note, it would be helpful to have some extra debug text in VK_LOADER_DEBUG=all to indicate which types of manifests (implicit layer/explicit layer/driver) were being searched and which env vars are being included or not in each one. I got myself confused while digging in because when enabling that I saw it was searching the path I wanted but there are multiple searches listed so crucially it wasn't searching some of the time.

Is this specific to a single platform? I encountered this on linux but I think it probably applies everywhere.