Compatible with Srclib

Srclib-java can now be used with srclib in both the Dockerfile and Program invocation methods. Currently scans for maven artifact files, defined in pom.xml's. Cannot be used in site yet due to lack of a formatter, but works with editor plugins.
I think this should be Artifact, not MavenArtifact, because non-Maven users can depend on Maven JARs without caring that they were originally created by Maven. There's nothing Maven-specific about them. Just "JavaArtifact" or probably just "java" would work (since this is how virtually everything is distributed in Javaland).