Better diagnostic for relative paths in visibilities

[bb1yd]

Fixes #118878

Note that the compiler suggest pub(in super) instead of pub(super), because we can only get the span of the path. in super is valid but tell me if there are better ways to do this :)

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;
   |
`

[rustbot]

Some changes occurred in diagnostic error codes

cc @GuillaumeGomez

[rustbot]

r? @dingxiangfei2009

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