[mvn_build] Add executed command into output

This makes debugging and reproducing easier
[mvn_build] Output commands on stderr

Also flush stderr before executing xmvn
[test] don't fail on scripts with future imports
[test] print stderr on test failures
Add xmvn-launcher to ivy-local-classpath
Fix POM path in generated XMvn reactor file
Fix unit test for mvn_artifact.py
Remove aether-api from ivy-local classpath