filter the messages during slushy::slushy_sync()?

[bzkrouse]

Previously, we waited until renv::restore() was finished to print messages, as it was an easy way to control what was being conveyed to the user.
In the most recent release, we are printing renv::restore() directly in case it takes a long time (i.e. packages not available in a cache).

However, maybe there is a middle ground where we can modify the messages in real time so they are more aligned to the messages created by try_install.