PackageCallbacks: fixed progress reporting

- Progress overflow was caused by missing stage count (`Progress` module counts stages as additional steps)
- Fixes `Progress bar has only 100 steps, not 101` error in `y2log`
- 3.1.113
I do not get it much, so now we have 100% of job plus some stages? I think it is quite unintuitive and deserve bigger comment with explanation why we need it.