Resolve dependencies on JOSM plugins that are neither in SVN nor Nexus

Currently, when you add a JOSM plugin as required plugin, but that plugin is not in the SVN repo and also not published on the JOSM Nexus instance, that required plugin can not be resolved.

There should be the possibility to add those plugins as required plugins. Probably https://josm.openstreetmap.de/plugin should be parsed to get the download URL for those.

See josm#16558 for the original report.