Update mvn_artifact.py tests to XMvn 2.x

XMvn 2.x uses simplified reactor installation.  There is no separate
field for raw POM path.  Instead POM files are treated as standalone
artifacts.
[mvn_artifact] Reflect changes in XMvn 2.0
[mvn_artifact] Formatting
[test] Fix failing test for mvn_artifact

Related to XMvn 2.0 changes
[maven.req] try to provide more helpful error messages
[maven.req] do not print the exception twice
Generate compat version for pom artifacts
Add definition of %_ivyxmldir macro
[fedora-review] Add external_plugin to silence f-r warning
Refresh metadata from central