fixed NPE.

classifier can be null, so classifier.intern() will cause NPE. Since I introduce "String intern(String)", using it everywhere else for being denfensive
[JENKINS-22354]

MavenArtifact class is loaded into Maven process, so don't drag too many classes dependencies into it.

Run class refers to a large number of classes, and in particular this code forces XStream instantiation which drags in quite a few number of classes