`.onUnload()` is no longer called

[hadley]

Probably because of the changes in 2d28e266bf111b3e49a5e1a473b053c7104d5459 where we no longer call unloadNamespace(). So now we should call .onUnload() if present, and similarly handle the onUnload hook.

[lionel-]

hmm, one problem I can see with this is that we try to preserve stale namespaces in a working state since it's so easy to run into dangling references, for example via an imports env populated before a load_all() call.

So it depends on what conventions should apply to the unload hook. Is it UB to call into a namespace that has run this hook, or should the package gracefully handle this situation?

[DavisVaughan]

Not calling .onUnload() was intentional according to https://github.com/r-lib/pkgload/issues/148#issuecomment-790910702