A member of Xojo team gave me the solution with Declare, but I think a FolderItem.IsPackage would be welcome.
There are many package ( . app , .rtfd and so on) which may be considered as file by our program (when we through each item of a folder).
When the Lib will be deprecated, I will have to ask again help to find news Declare instructions.
Not all declares get ever deprecated; there are still declares from earlier version of MacOS X (possibly since the first release) used these days.
Asking for a not cross-platform function may not always succeed in Xojo (look at a lot of OS features not available directly because they target a single platform).
As I understand it, the basic rule of a package is a folder with a file extension that is registered on the system.
Yes, I believe it’s true. LaunchServices knows extensions that make a folder being a bundle and registers them like other documents (e.g. upon app’s first launch/discovery).