parameter decorator: handle preset files as resources

[bertsky]

When we drafted the resource handling, one of the use cases was installing preset files, i.e. preconfigured (if incomplete) json parameter sets that have a discernable name and can be used as value for --parameter|-p but without the directory path.

That latter part is still missing. So we still need to modify … https://github.com/OCR-D/core/blob/0713e0e1c2fd11242957c3a51c974a1048dba997/ocrd/ocrd/decorators/parameter_option.py#L7-L14 … such that known resource filenames are tried first.

[bertsky]

For example, we can currently do ocrd-preprocess-image -L and -C param_im6convert-denoise-raw.json but not -p param_im6convert-denoise-raw.json.