Extension could be marked as core but not available at runtime

See MARKDOWN-9 for an example use case.

Here's the problem:

• XWiki's extension scanner uses reflections to find all JARs in the classloader, going up till the top parent CL. This means, for example, adding the jars in jetty/lib as core extensions.
• At runtime when loading a class, the Jetty's WebAppClassLoader is used and this CL isolates the server classes (ie the classes in jetty's lib directory), see http://www.eclipse.org/jetty/documentation/9.4.x/jetty-classloading.html

Thus if you have an extension that uses a class that is inside a jar located in jetty's lib directory, this jar won't be installed as an extension and the class won't be found at runtime.