Hello this is a little tweak to allow Minecraft ClassLoader to search for Plugin class. This coremod is a POC who i have use since a long time on my own server so if you find issue related to your server feel free to open an issue look at HowWriteMyISSUE.md
You need to install this coremod in you server ONLY To allow this coremod to work correctly you need to replace in your library net.minecraft.launchwrapper.1.12 by this one
You can call plugins from FMLServerStartedEvent and after before this event plugin are not loaded yet