Currently we only publish the meta-data, what works for most of the time
if a junit bundle is missing, but for the case where the bundle name
does not match the artifact key (e.g. junit4/hamcrest) this leads to
Tycho not find the file and injecting a dummy file that can confuse
maven-surefire-plugin.

This now adds a new IRawArtifactFileProvider provider for such missing
bundles that is asked as last resort if no other part of the system can
answer for a file.

(cherry picked from commit fb32453)