core: list_all_resources does not handle subdirectories well

In ocrd_utils.list_all_resources (used by a processor’s --list-resources and resmgr’s --list-installed), entries that are themselves directories do not get traversed recursively.

That’s okay if the processor takes directory resources (e.g. ocrd-calamari-recognize’s h5/json checkpoint dirs), but not if its resources are files in subdirectories (e.g. ocrd-tesserocr-recognize’s script/*.traineddata or configs/*).

So IMO a better behaviour would be to list all such directories recursively.