support conversion to qualified usage

[ericphanson]

maybe we could support an experimental source-rewriting feature where we replace implicitly imported names with qualified usages by rewriting the source code. We have the very localized info of where the usages are thanks to the precision of JuliaSyntax so I think this would be doable. It's a bit scary to touch users' files though. If we wanted, we could actually introspect the git state and ensure we are in a non-dirty git repo for safety, before updating any files.