Log the origin of the manifest search paths (system, env-var, settings file, etc)

[charles-lunarg]

**What enhancement are you suggesting for the Vulkan Loader? Please describe in detail. **

#1394 has a side note that it would make debugging easier if the log contained where the paths used to search for manifests came from, whether that is the default system paths, environment variables, override layer, layer settings file, or any other possible location.

Additional context

Original request:

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.