Do not rely on current working directory when running the steward binary

[OndraM]

Currently, steward must be run from root directory of the project and with command vendor/bin/steward. Otherwise things stops working, because some of the directories are treated relatively from the current working directory - path to listeners, default path to tests etc.

We may change the behavior to detect the root dir automatically, so this is not confusing for newcomers and do not cause unexpected behavior.