Processor.resolve_resource ignores without-extension

[Robert Sachunsky]

AFAICS the ocrd-tool.json's parameter_usage is nowhere used (except in a log message), and resolving resources without-extension does not work.

It looks like instead of os.path.exists, in this case one would need to (somehow) look up the typical extension(s) from the resource list and then go look for them in the filesystem.