Only the `Sys()` value from os.FileInfo is kept as field `sys` to
support Windows. The os.FileInfo removal ensures that for values like
`ModTime` that existed in both data structures there's no more confusion
which value is actually used.
The actual implementation still relies on file paths, but with the
abstraction layer in place, an FS implementation can ensure atomic file
accesses in the future.
We don't need `error` here: the only existing implementation
of `ErrorHandler` always call `Backup.Error` and all
implementations of `Backup.Error` always return nil.
