implemented optional dependencies

Plugin authors could now define optional dependencies to other plugins in their pom.
Extensions which are using classes from optional dependencies must specify this
with the "requires" attribute of the extension annotation.
Extensions with "requires" attribute are not installed if one of the specified plugins,
is not installed.

scm-webapp/src/main/java/sonia/scm/plugin/DefaultPluginLoader.java

@@ -99,7 +99,7 @@ public class DefaultPluginLoader implements PluginLoader
 
       modules = getInstalled(parent, context, PATH_MODULECONFIG);
 
-      collector = new ExtensionCollector(Iterables.concat(modules, unwrap()));
+      collector = new ExtensionCollector(parent, modules, installedPlugins);
       extensionProcessor = new DefaultExtensionProcessor(collector);
     }
     catch (IOException | JAXBException ex)
@@ -170,19 +170,6 @@ public class DefaultPluginLoader implements PluginLoader
     return uberWebResourceLoader;
   }
 
-  //~--- methods --------------------------------------------------------------
-
-  /**
-   * Method description
-   *
-   *
-   * @return
-   */
-  private Iterable<InstalledPluginDescriptor> unwrap()
-  {
-    return PluginsInternal.unwrap(installedPlugins);
-  }
-
   //~--- get methods ----------------------------------------------------------
 
   /**