Clearer status information from opam package as to which Visual Studio was selected

[dra27]

It would be good to have post-messages entries making it clear which Visual Studio instance is being used (this could be displayed regardless of whether the user has more than one Visual Studio installation). It would also be good to display warnings if versions were ignored along with a reason (e.g. a VS 2019 instance selected because of a VS 2022 instance doesn't have all the required tools).