New method for project artifacts

Introduce a new method on API Project object that exposes "main artifacts" of project. The method can return 1 or 2 artifacts (never less than 1 and never more than 2). These are NOT the attached artifacts! (see ProjectManager for that).

In short: projects with packaging "pom", "bom" (that produce only one artifact) will on this method return 1 artifact, while projects for example with packaging "jar" will return 2 artifacts (as they produce, install, deploy two artifacts, the POM and the JAR/WAR/etc).