Unfortunately as far as I can tell the only thing Logic provides is an option to rescan a selection of AUs that may have failed.
They don’t seem to offer any method to rescan without clearing com.apple.audiounits.cache, and they don’t let you specify a specific directory to rescan either.
Also what version of OSX are you on? There is an issue with AudioUnits on OSX High Sierra if you’re running that version:
I’m on High Sierra and most of the time I have to do that “renaming” trick that’s explained in that thread