Use visibility information to narrow down search scopes

[mkaput]

We can further expand SearchScopes optimization in find references implementation, by making use of item visibilities to narrow down searched file sets.

For example, if an item is not public outside a crate, we do not have to search for it in other crates. The same happens if an item is not public outside the current file: we only have to look in this file and submodule tree.