Fix and refactor repo-pkgs command

It was bad idea to reuse other commands. The implementation was ugly and the documentation wasn't clear. I'm sorry for that mistake.

These patches mainly refactor the repo-pkgs command. I've implemented its sub-commands as new classes and improved their documentation. I've also added some tests to make sure the commands align with the documentation. It turned out that ```repo-pkgs info installed``` did not work. As a consequence, it also turned out that the ```installPkgs``` and ```updatePkgs``` methods are no longer needed.

Please note that this is good opportunity to also drop the ```repo-pkgs install @Group``` functionality, because my previous arguments aren't valid now (implementing it, it does not simplify documentation nor implementation).

We have to release new version of dnf-plugins-core before releasing dnf, because of the ```installPkgs``` removal.