Get plugin source status

Reports whether a plugin from outside the curated plugin list has ever been installed on this system, at one of two confidence levels: "unknown" -- it definitely has. "unverified" -- one may have been: something was installed while FPP could not reach the plugin list to check it, and the list still isn't cached, so the question is still open. "none" -- nothing to report. Answered entirely from settings plus an already-cached plugin list. No network call, no plugin-directory scan: this is on the health check's path and has to be instant even on a box with no internet.