SLING-8660 - Add optional support for precompiled scripts in the HTL engine

* documented why the PrecompiledUnitManager is not by default registered as a service
diff --git a/src/main/java/org/apache/sling/scripting/sightly/impl/engine/precompiled/PrecompiledUnitManager.java b/src/main/java/org/apache/sling/scripting/sightly/impl/engine/precompiled/PrecompiledUnitManager.java
index fcdfc36..f880a1d 100644
--- a/src/main/java/org/apache/sling/scripting/sightly/impl/engine/precompiled/PrecompiledUnitManager.java
+++ b/src/main/java/org/apache/sling/scripting/sightly/impl/engine/precompiled/PrecompiledUnitManager.java
@@ -33,7 +33,13 @@
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
-@Component(immediate = true)
+@Component(
+        immediate = true,
+        service = {}
+        /*
+         * this component will register itself as a service only if the org.apache.sling.scripting.bundle.tracker API is present
+         */
+)
 public class PrecompiledUnitManager {
 
     private static final Logger LOGGER = LoggerFactory.getLogger(PrecompiledUnitManager.class);