maven_depmap.py: Fix rhbz#1007203

specify artifact extension in depmap if other than "jar"
Improve regexp for backreference counting in artifact validation
Move validating of backreferences and wildcards into XMvnConfig

This way we have everything neatly in one place
depmap: fail if depmap does not provide any artifacts

Maybe this has some use cases, but will prevent broken fragment files from
reaching packages. We might revisit in future
Add TODO for pom_editor.sh