common/src/main/java/org/jboss/pnc/common/util/StreamCollectors.java
public static <T> Collector<Collection<T>, List<T>, List<T>> toFlatList() {
        return Collector.of(
                ArrayList::new,
                List::addAll,
                (left, right) -> {
                    left.addAll(right);
                    return left;
                });
    }
public static <T> Collector<Collection<T>, List<T>, List<T>> toFlatList() {
        return Collector.of(
                ArrayList::new,
                List::addAll,
                (left, right) -> {
                    left.addAll(right);
                    return left;
                });
    }
Flattening collector.
Look at StreamCollectorsTest for example usage


@param <T> type of elements in the collections
@return flattened list of elements from all of the collections