Suggest public re-exports when a private module makes an import path inaccessible

[jakubadamw]

This is an attempt at solving #13065.

When a use path fails because it passes through a private module (E0603), and a public re-export of the target item exists elsewhere, the compiler will now suggest importing through that re-export instead.

For example, given:

mod outer {
    pub use self::inner::MyStruct;
    mod inner {
        pub struct MyStruct;
    }
}

use outer::inner::MyStruct; // error: module `inner` is private

the compiler will now suggest use outer::MyStruct; - the publicly accessible path - rather than just pointing at the private module definition and leaving the user to figure out the alternative.

When possible, relative paths are suggested, including those using super (currently capped at a maximum of one super path item).

The newly added test is parametrised by editions because of the change in behaviour around crate::-prefixed imports in edition 2018. Perhaps that’s an overkill – I’ll be happy to remove the variations for editions 2021 and 2024.

Closes #13065.

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