check packages in YAML

[brandmaier]

Currently, if there is a typo or an invalid package in the list of dependent packages, this will show up as an error only relatively late in the build process of the container. It would be very helpful if repro would initially check whether the given package names are available on CRAN at all (e.g., when the command reproduce() is run).

[brandmaier]

The check should really happen when 'automate()' is called, I guess.