chore(model): refactor copier for more flatness (#10094)

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).

lib/model/folder_sendrecv_test.go

@@ -14,6 +14,7 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"runtime/pprof"
 	"strconv"
 	"strings"
 	"testing"
@@ -680,6 +681,17 @@ func TestCopyOwner(t *testing.T) {
 		expGroup = 5678
 	)
 
+	// This test hung on a regression, taking a long time to fail - speed that up.
+	ctx, cancel := context.WithTimeout(context.Background(), 2*time.Second)
+	defer cancel()
+	go func() {
+		<-ctx.Done()
+		if errors.Is(ctx.Err(), context.DeadlineExceeded) {
+			pprof.Lookup("goroutine").WriteTo(os.Stdout, 2)
+			panic("timed out before test finished")
+		}
+	}()
+
 	// Set up a folder with the CopyParentOwner bit and backed by a fake
 	// filesystem.