Add restart for package not found errors

[lionel-]

check_installed() signals error conditions of class rlib_error_package_not_found. The error includes pkg and version fields. They are vectorised and may include several packages.

The error is signalled with a rlib_restart_package_not_found restart on the stack to allow handlers to install the required packages. To do so, add a calling handler for rlib_error_package_not_found, install the required packages, and invoke the restart without arguments. This restarts the check from scratch.

The condition is not signalled in non-interactive sessions, in the restarting case, or if the rlib_restart_package_not_found user option is set to FALSE.

For now the ctor and signal functions for the package-not-found errors are not exported because the error class and signalling protocol are rather intertwined.

[gaborcsardi]

Cool! I'll add support for this in pak, and we should probably add links to the docs, both ways.

[gaborcsardi]

@lionel- Actually, wait a sec before merging.

[gaborcsardi]

I just checked that requireNamespace() in is_installed() does not throw a packageNotFoundError condition currently, only loadNamespace() does. So that is good, because pak will also catch packageNotFoundError.

However R-core is considering throwing this condition from find.package(), in which case is_installed() will also throw packageNotFoundError, possibly, depending on how they implement this. Hopefully they'll do it in a way that we can still use requireNamespace() to check for installed packages.

I think we don't need to do anything special about this just yet, just wanted to note it here.

So I think this PR is good to merge. :)

[lionel-]

hmm chances are they won't signal a condition on requireNamespace() because if a restarting handler is attached that would probably be quite distracting for users, in the case of packages with lots of Suggests features. Thanks for the review!