lib prjxray: less dependent on .json files

I need to run tools against .db files early on during db construction. However, the library is currently written assuming the entire .db has been compiled. This precludes a lot of usage since it requires the .json files from 074-dump_all, which isn't ran until very late and takes a long time

Other: to improve usability, we should make a few small examples on how to do a few common tasks