loader: Don't add duplicate JSON files from Khronos registry path.

loader: Don't add duplicate JSON files from Khronos registry path.

To make sure we avoid enumerating old JSON files/drivers that might be present
in the non-device specific area of the registry when a newer device specific
JSON file is present, do a check before adding.
Find the file name, without path, of the JSON file found in the non-device
specific registry location. If the same JSON file name is already found in
the list, don't add it again.