Reduce visibility of misleading install function

This install function might be mistaken to really install this plugin,
whereas it only sets its internal status and does not really do smth.

scm-core/src/main/java/sonia/scm/plugin/AvailablePlugin.java

@@ -25,7 +25,7 @@ public class AvailablePlugin implements Plugin {
     return pending;
   }
 
-  public AvailablePlugin install() {
+  AvailablePlugin install() {
     Preconditions.checkState(!pending, "installation is already pending");
     return new AvailablePlugin(pluginDescriptor, true);
   }