Add Maven Central backend

Adds support for having Maven projects specified by groupId:artifactId
you probably meant plexus:plexus-compiler. It's Maven's format for specifying artifacts and the groupId:artifactId string is generated from project's pom.xml, it cannot be just plexus-compiler (and even if it could, I still don't see any clash, the URL will be different). Even though in normal language you'll probably call it just by its artifactId, artifactId itself is not enough to identify project. There can be (and often are) multiple projects with the same artifactId (usually forks or different implementations of some specifications).