ECMQmlModule: Use latest version from individual sources as default

In Kirigami, we have 2.0 for the version set when adding the QML module.
But the individual sources have version numbers up to 2.20.

In order for ecm_find_qml_module to work properly, we need the 2.20
version in the qmldir file as a comment.
To avoid having those versions go out of sync, we can use the latest
version from the sources unless we explicitly specify one.