SPIGOT-6822: Ensure library loader has same parent as plugin loader

This behaviour may need to be revisted in future to achieve greater isolation of plugins/libraries

By: md_5 <git@md-5.net>

paper-api/src/main/java/org/bukkit/plugin/java/LibraryLoader.java

@@ -121,7 +121,7 @@ class LibraryLoader
             } );
         }
 
-        URLClassLoader loader = new URLClassLoader( jarFiles.toArray( new URL[ jarFiles.size() ] ) );
+        URLClassLoader loader = new URLClassLoader( jarFiles.toArray( new URL[ jarFiles.size() ] ), getClass().getClassLoader() );
 
         return loader;
     }