cct plugin: Adjust append_sources to be optional
cct plugin: Adjust append_sources to be optional

Not all cct modules will provide a sources file. Check and if there
isn't one return quietly rather than throwing an exception.

goldmann/dogen,jboss-container-images/concreate,jboss-container-images/concreate,jboss-dockerfiles/dogen,jboss-dockerfiles/dogen,goldmann/dogen,jboss-container-images/concreate,jboss-dockerfiles/dogen,goldmann/dogen