Plugin handling updating of config files

[MatthieuDartiailh]

When a config file is loaded and found to use older packages than the ones available, we may need to update part of it (and save after update) or abort the loading. Aborting is the choice of the user and should be left to the user, trying to update the file requires to have tools able to do the update.
The updating mechanism should require contributed updaters that work on the config object and specify the old version and the new version (ideally those two should be two contiguous releases) and collect all the updaters that need to be applied (potentially chain multiple ones).
This system also requires to be able to report update failure to provide a meaningful error message.

Some details may be missing so this remains at the discussion stage for the time being.