Xcode autocomplete does not work in Sources folder of Swift playgrounds
In order to enable autocompletion, you can embed your Playground in a regular Xcode project (e.g. an iOS application). I recommend creating a dummy project for that purpose. Simply drag and drop your playground in this dummy project and make sure to check "Add to target".
Then you can navigate to
Target -> Build Phases -> Compile Sources -> + -> Add other
and add all the files from your source folder. Please note, that you don't need to actually copy the files, a reference is enough for this purpose.
After this process all your source files are built against this dummy target and you can use autocompletion as usual. As far as I know, this is the best practice for debugging Playgrounds right now. Anyway I am curious, if there is an easier way to achieve that.