Set scope of Plexus dependency to provided

XMvn Core code doesn't depend on Plexus container, only unit tests use
Plexus.
Add test dependency on junit
Migrate from Plexus to Sisu

Sisu provides interface compatible to Plexus Container.  The migration
will allow to gradually migrate from Plexus component annotations to
JSR-330.

See: http://maven.apache.org/maven-jsr330.html
[buiddep-mojo] Don't print BR java-devel if java version is <= 1.5