Do not allow plugins to make multiple instances of their main class.

Prevents foot shooting, eg #2772

proxy/src/test/java/net/md_5/bungee/api/plugin/DummyPlugin.java

@@ -1,11 +1,12 @@
 package net.md_5.bungee.api.plugin;
 
-import lombok.AccessLevel;
-import lombok.NoArgsConstructor;
-
-@NoArgsConstructor(access = AccessLevel.PRIVATE)
-public class DummyPlugin extends Plugin
+public final class DummyPlugin extends Plugin
 {
 
     public static final DummyPlugin INSTANCE = new DummyPlugin();
+
+    private DummyPlugin()
+    {
+        super( null, null );
+    }
 }