As far as I know, import module/other script in the package install script file is not supported.
I tried on the recent master branch build (3.2.0), that give output Warning: QFile::remove: Empty or null file name and an exception thrown on the line that execute import.
I am using import syntax that described on the QJSEngine docs and the one that in old QT 5.7 docs.
In my use case I would like to integrate the installer with our own license management and validate through http request. So as a user I would like to have import module/other scripts feature to improve modularity and reusability that can also affected to easier maintanance and update.

Thank you