resmgr list-installed only knows about 3 processors with preconfigured resources

[bertsky]

But surely, such a user database was at least created by that call. And if you did not run any list-available prior to that, then that database would be just a mirror of the distributed ocrd/resource_list.yml (hence only those 3 processors).

So we just uncovered another serious bug: initialisation does not search the PATH for ocrd-* executables, only list-available does. But without these database entries, list-installed never even attempts to look for other executables!

Originally posted by @bertsky in https://github.com/OCR-D/core/pull/1246#discussion_r1665951656