`fdroid scanner --json` for machine readable output

Issuebot now does everything in JSON, so this makes fdroid scanner output JSON for easy use there.

This also throws in a small check that follows up on @uniqx's yamllint !721 (merged). A srclib file will always produce a dict as the container, so this checks the output is a dict. A 0 length file or any there data structure, e.g. a list, will cause an error.