Do not check for packages in inst-sys

- bnc#888130
- Installing checking for or installing new packages in inst-sys doesn't work anyway
- For now we just log it and skip it
- In the future, enhanced methods would make sense, see `Ideas`
- BTW, the test covers as much as I wanted now, it doesn't describe the whole method but I will not extend it now - it needs more time and a bit of refactoring there first