Extendscript: Package command

Thanks both.
So I ended up going the route of scooping the placed items and placing them in the export folder, then updating the links. For reference:

function exportLinkedItems(doc, folderPath){
    var placedItems = doc.placedItems;
    for (var i = 0; i < placedItems.length; i++){
	var placedItem = placedItems[i];
	var linkedFile = placedItem.file;
	if (linkedFile === null){
		continue;
	}
	var newPath = folderPath + "/" + linkedFile.name;
	linkedFile.copy(newPath);
	var newFile = File(newPath);
	placedItem.file = newFile;
    }
}

I'm not sure of the exact syntax but try using the app.executeMenuCommand. 

You can try

app.executeMenuCommand("Package Menu Item");

but it will prompt to the user, with regular Package prompt.

Thanks to @Ten-A we can also include this so it's easier to execute menu commands.

If you want to do it without prompts, then it should be doable with getting all links (placedItems), and saving them in specific folder.