keep GraalPyExtension packages as a Set instead of a List

Author: tomasstupka

graalpython/graalpy-gradle-plugin/src/main/java/org/graalvm/python/dsl/GraalPyExtension.java

@@ -43,7 +43,7 @@
 import org.gradle.api.Action;
 import org.gradle.api.file.DirectoryProperty;
 import org.gradle.api.provider.ListProperty;
-import org.gradle.api.provider.Property;
+import org.gradle.api.provider.SetProperty;
 import org.gradle.api.tasks.Nested;
 
 public interface GraalPyExtension {
@@ -57,7 +57,7 @@ public interface GraalPyExtension {
     /**
      * Determines third party python packages to be installed for graalpy usage.
      */
-    ListProperty<String> getPackages();
+    SetProperty<String> getPackages();
 
     /**
      * Determines what parts of graalpy stdlib are supposed to be available for graalpy.