[MNG-6275] Maven Embedder compatible fix

- Embedded sets up a classloader to mimic the boot classpath based on parsing the classworldConf
- This 'bootLoader' is then installed as the context classloader in the current thread
- By default, a running JVM will start with a thread context classloader of 'null' to indicate the
  system classloader should be used
- This change switches to follow the 'correct' behaviour of following the context classloader
  and falling back to the system class loader only in the case where the context classloader is
  null.
- For running from the CLI, we expect that the thread context classloader will always be null
  so this should be as before
- For running from embedder, we expect that the thread context classloader will have been correctly
  configured, so this should behave as intended

maven-core/src/main/java/org/apache/maven/classrealm/DefaultClassRealmManager.java

@@ -120,7 +120,9 @@ private ClassRealm newRealm( String id )
             {
                 try
                 {
-                    ClassRealm classRealm = world.newRealm( realmId, PARENT_CLASSLOADER );
+                    ClassLoader contextClassLoader = Thread.currentThread().getContextClassLoader();
+                    ClassRealm classRealm = world.newRealm( realmId, contextClassLoader == null
+                                    ? PARENT_CLASSLOADER : contextClassLoader );
 
                     if ( logger.isDebugEnabled() )
                     {