Use a special plugin class loader to resolve classes from other plugins.

api/src/main/java/net/md_5/bungee/api/plugin/PluginClassloader.java

@@ -0,0 +1,33 @@
+package net.md_5.bungee.api.plugin;
+
+import java.net.URL;
+import java.net.URLClassLoader;
+import java.util.HashSet;
+import java.util.Set;
+
+public class PluginClassloader extends URLClassLoader
+{
+
+    private static final Set<PluginClassloader> allLoaders = new HashSet<>();
+
+    public PluginClassloader(URL[] urls)
+    {
+        super(urls);
+        allLoaders.add(this);
+    }
+
+    @Override
+    protected Class<?> findClass(String name) throws ClassNotFoundException
+    {
+        for (PluginClassloader loader : allLoaders)
+        {
+            try
+            {
+                return loader.loadClass(name);
+            } catch (ClassNotFoundException ex)
+            {
+            }
+        }
+        throw new ClassNotFoundException();
+    }
+}

api/src/main/java/net/md_5/bungee/api/plugin/PluginManager.java

@@ -158,7 +158,7 @@ public class PluginManager
             try (InputStream in = jar.getInputStream(pdf))
             {
                 PluginDescription desc = yaml.loadAs(in, PluginDescription.class);
-                URLClassLoader loader = new URLClassLoader(new URL[]
+                URLClassLoader loader = new PluginClassloader(new URL[]
                         {
                             file.toURI().toURL()
                         });