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

**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.

Start by tracing the Vulkan Loader's VK_LOADER_DEBUG=all manifest-search logging; the issue names no source files, tests, or entry points. Completion means the debug output identifies implicit-layer, explicit-layer, and driver searches, plus whether each path came from system defaults, environment variables, overrides, or settings files.

Written by the indexing model from the issue text.