PrimeFaces application fails with classloader errors on EAP 8

Issue

• We're upgrading our PrimeFaces application from EAP 7 to EAP 8 and see classloader errors like below in the application start up:

WARN  [org.jboss.modules.define] (MSC service thread 1-1) Failed to define class org.primefaces.extensions.application.PostConstructApplicationEventListener in Module "deployment.app.war" from Service Module Loader: java.lang.NoClassDefFoundError: Failed to link org/primefaces/extensions/application/PostConstructApplicationEventListener (Module "deployment.app.war" from Service Module Loader): javax/faces/event/SystemEventListener
...
WARN  [org.jboss.modules.define] (MSC service thread 1-1) Failed to define class org.primefaces.extensions.application.PrimeFacesExtensionsResourceHandler in Module "deployment.app.war" from Service Module Loader: java.lang.NoClassDefFoundError: Failed to link org/primefaces/extensions/application/PrimeFacesExtensionsResourceHandler (Module "deployment.app.war" from Service Module Loader): javax/faces/application/ResourceHandlerWrapper
...
Caused by: java.lang.ClassNotFoundException: javax.faces.webapp.FacesServlet from [Module "deployment.app.war" from Service Module Loader]