Extend version mismatch warning to `$packages`

[sebffischer]

Why do we only compare the mlr3 version of the learner with the current version when calling$predict() ?

[mllg]

First step would be to replace the $packages slot with a named list of packageVersion(). The main operations on the packages slot are: require_namespaces(), union() and print()/paste(). It is not cleat to me what to do in the case of an union() if the package versions differ.