Flattened the copier code more. Also removing and moving some
parameters/return values to simplify things. Generally rely less on
return values, e.g. by handling errors right away and using `state` to
do the right thing (e.g. abort on failure).
Supposed to be a refactor without any behaviour changes, except for
fixing a tiny regression on folder order: We used to try copying from
the same folder first, but lost that property at some point (also sent a
PR fixing only that, I'd merge that first making this refactor only).
821d6f43ac