Scripting, maven: use getArtifacts to get the plugin's runtime classpath

..to correctly pass the dependencies in the ExecuteKotlinScriptMojo.
#KT-50306 fixed

Also add a test that a script has access to plugin-configured
dependencies.
If plugin/dependencies is populated, the extra artifacts should be
made available when the script is run. This test uses 'junit' simply
because it's commonly-used, but not already present, or shaded as part
of another artifact.

libraries/tools/kotlin-maven-plugin/src/main/java/org/jetbrains/kotlin/maven/ExecuteKotlinScriptMojo.java

@@ -247,7 +247,7 @@ public class ExecuteKotlinScriptMojo extends AbstractMojo {
     }
 
     private List<File> getThisPluginDependencies() {
-        return plugin.getDependencies().stream().map(this::getDependencyFile).collect(Collectors.toList());
+        return plugin.getArtifacts().stream().map(this::getArtifactFile).collect(Collectors.toList());
     }
 
     private File getThisPluginAsDependency() {