146a7e4bd76 adds library support, like on BungeeCord.
This feature works brilliantly on BungeeCord, however, it does not on Spigot:
If a plugin loads a class of a library it is dependent on a ClassCastException is thrown.
This is because Spigot expects the Classloader of a class loaded from another PluginClassloader to be an instance of PluginClassLoader as well see:
 
https://hub.spigotmc.org/stash/projects/SPIGOT/repos/bukkit/browse/src/main/java/org/bukkit/plugin/java/PluginClassLoader.java#119
 
If this check would only apply if the Classloader of the given class is a PluginClassloader the classloading would work as well as on BungeeCord