Restored the old PluginLoader as AddonLoader

Introduce a new `IAEAddon` interface, which has to be implemented and
annotated wit `@AEAddon` to obtain access to our public API.

src/api/java/appeng/api/AEAddon.java

@@ -26,12 +26,16 @@ import java.lang.annotation.Target;
 
 /**
  * Use this annotation on a class in your Mod to have it instantiated during the
- * initialization phase of Applied Energistics. AE expects your class to have a
- * single constructor and can supply certain arguments to your constructor using
- * dependency injection.
+ * initialization phase of Applied Energistics.
+ * 
+ * The class also needs to implement {@link IAEAddon}.
+ * 
+ * AE expects your class to have a single constructor without any parameters.
+ * 
+ * This is the only way to get access to the public {@link IAppEngApi} instance.
  */
 @Target(ElementType.TYPE)
-@Retention(RetentionPolicy.RUNTIME)
+@Retention(RetentionPolicy.CLASS)
 @Documented
-public @interface AEPlugin {
+public @interface AEAddon {
 }

src/api/java/appeng/api/AEInjectable.java

@@ -25,7 +25,7 @@ import java.lang.annotation.Target;
 
 /**
  * Marks interfaces that can be used as injectable constructor arguments for an
- * {@link AEPlugin}.
+ * {@link AEAddon}.
  */
 @Target(ElementType.TYPE)
 @Retention(RetentionPolicy.RUNTIME)

src/api/java/appeng/api/IAEAddon.java

@@ -0,0 +1,50 @@
+/*
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2020 TeamAppliedEnergistics
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to
+ * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of
+ * the Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER
+ * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+package appeng.api;
+
+import net.minecraftforge.fml.event.lifecycle.FMLCommonSetupEvent;
+
+/**
+ * Every AE2 addon requiring access to {@link IAppEngApi}, needs to provide at
+ * least one class implementing this interface.
+ * 
+ * Further it requires the class to be annotated with {@link AEAddon}.
+ * 
+ */
+public interface IAEAddon {
+
+    /**
+     * This gets called once the API is successfully constructed and ready to be
+     * used.
+     * 
+     * For now this happens during {@link FMLCommonSetupEvent}.
+     * 
+     * Each addon is responsible to maintain a reference to {@link IAppEngApi} for
+     * future use. Otherwise there is no alternative to access it later.
+     * 
+     * @param api The API instance when ready.
+     */
+    void onAPIAvailable(IAppEngApi api);
+
+}