Print warning if dnf5 is used to manage the system

[kontura]

On transaction operations we could check that dnf5 system state nevras.toml is present/non empty (its what libdnf5::dnf4converter does) and if true print a warning that it is not recommended to manage one system with both dnf4 and dnf5.

[ppisar]

Do you want to hard code a path to nevras.tml in DNF4 sources, or parse dnf5 --dump-main-config output?

[kontura]

I did not consider parsing the dnf5 --dump-main-config output, that is an interesting idea.

On the other hand perhaps it would be enough to hard code it. Its simpler and if someone moves the system state I would assume they are a more advanced user.