Add a 'plugins' source set to the game module

Creates a new sourceset for server plugins to support IDE integration
with plugin code.  Additionally moves plugins to game/data/plugins and
contains some gradle related fixes (remove java plugin from root project
and fix exec task dependency).

game/data/plugins/stub.kt

@@ -0,0 +1,20 @@
+/**
+ * NOTE: This file is a stub, intended only for use within an IDE.  It should be updated
+ * each time [org.apollo.game.plugin.kotlin.KotlinPluginScript] has a new method added to it.
+ *
+ * Until IntelliJ IDEA starts to support ScriptTemplateDefinitions this is
+ * required to resolve references within plugin code.
+ */
+
+import org.apollo.game.model.World
+import org.apollo.game.plugin.PluginContext
+import org.apollo.game.plugin.kotlin.KotlinMessageHandler
+import org.apollo.net.message.Message
+import kotlin.reflect.KClass
+
+val world: World = null!!
+var context: PluginContext = null!!
+
+fun <T : Message> on(type: () -> KClass<T>): KotlinMessageHandler<T> {
+    null!!
+}