Use "pack file" instead of "data file" (#2885)

- changed variable names, especially changed DataFile into PackFile
- changed in some comments
- always use "pack file" in docu

internal/backend/swift/swift.go

@@ -298,7 +298,7 @@ func (be *beSwift) IsNotExist(err error) bool {
 // It will not remove the container itself.
 func (be *beSwift) Delete(ctx context.Context) error {
 	alltypes := []restic.FileType{
-		restic.DataFile,
+		restic.PackFile,
 		restic.KeyFile,
 		restic.LockFile,
 		restic.SnapshotFile,