add a lint for `pub` items that are not reachable from crate root

[nikomatsakis]

As part of #44660, we need to create a lint for items that are declared as pub but are not reachable from the crate root (such items are advised to use crate visibility instead).

The lint should probably be allow-by-default for now, to start.

This issue needs precise mentoring instructions. There are some tips regarding adding "future compatibility" lints here in this forge article -- see the section "Issuing future compatibility warnings". We don't need a future compatibility lint here, so we can skip the stuff specific to that, but it does give some more general advice on how to make lints overall.

[nikomatsakis]

@zackmdavis mentioned that they were working on this