Better diagnostic for relative paths in visibilities - #162114

Fixes #118878

Note that the compiler suggest pub(in super) instead of pub(super), because we can only get the span of the path. It is possible to add another field in VisibilityKind::Restricted and pass a span of the entire parentheses for this diagnostic, but it will increase the size of the Item struct, not sure if it's worth doing.

I also change the original message from try to try adding a `crate` prefix because rustc stops inlining the suggestion when there are multiple, and a single word looks weird:

error[E0807]: relative paths are not supported in visibilities in 2018 edition or later
  --> $DIR/E0807.rs:6:5
   |
LL |     pub(in parent) struct Baz;
   |     ^^^^^^^^^^^^^^
   |
help: try
   |
LL |     pub(in crate::parent) struct Baz;
   |            +++++++
help: use `super` if you want this item to be visible in the parent module
   |
LL -     pub(in parent) struct Baz;
LL +     pub(in super) struct Baz;
   |
`