[feat] return list of copied files from copy utils

[pngwn]

Returns what files have been copied from the copy utilities provided to adapters. This way adapters can easily determine what files have been copied.

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[dummdidumm]

General thought: are there other things devs might want to have returned from this in the future? If yes, future-proof the API by returning an object with just the files property for now?

[pngwn]

This returns the source of the copied files. the only additional thing I can think of returning is the target they are copied to. That can easily be deduced by the consumer, as this is a straightforward recursive copy, but that is the only other thing that comes to mind.

If we wanted to do this then we could return and object of { source: Array<string> } which could later be expanded to { source: Array<string>, target: Array<string> }