Get plugin provenance status

Live-scans the plugin directory and classifies every currently-installed plugin into official/community/unknown (see ClassifyPluginProvenance()), combined with the sticky "ever installed" settings so plugins removed after this feature shipped still show up. A directory counts as "installed" even if incomplete/partially installed (e.g. missing pluginInfo.json) -- such a plugin can't be classified by srcURL/index lookup, so it falls into 'unknown'.