de-coupled "--check-library" from "information" severity and other dependencies

[firewave]

This is basically a proper fix for --check-library displaying things unlike the quick fix I did in 7c5015d54e422b02d58452d958f1c359056236c1 which introduced additional information and missingInclude ('information' includes that) checks and messages when using that flag. That does not make sense at all so I de-coupled those (as checks already were).

It was mentioned in some versions of the "documentation" that you should specify --enable=information as well but it was everything but clear.

On a side note I wonder if the cfg tests should also include --inconclusive in their flags.

[firewave]

This still requires checks in checkleakautovar.cpp and checkunusedvar.cpp adjusted as well as unit tests to check if those warnings are actually triggered. The message from CheckUnusedVar would not have been written at all even if both options specified since it also relies on style.