E0583: Improve error message to help with use vs mod confusion

[kornelski]

Feedback from a user forum thread. This code:

mod foo {
}

mod bar {
    mod foo;
}

currently suggests:

= help: name the file either foo.rs or foo/mod.rs inside the directory "src/bar"

This is awkwardly worded. It doesn't clearly explain that this syntax creates a new module. It doesn't guess that the user may have meant use crate::foo.

I suggest:

1. Improve wording of the message. "To create a new module crate::bar::foo, create "src/bar/foo.rs" file" (to keep it simple, without also suggesting 2015-edition path).

2. If there already is a module with that name, suggest use instead.

[kornelski]

I couldn't find a way to get path to the current module, or names of already parsed modules.

It seems that this info isn't explicitly tracked at such an early stage in the parsing, and I'm not sure if it's OK to add it.

[estebank]

I'm unambiguously in favor of 1 and tepidly positive about 2. The implementation of 2 might be difficult that early on.

[estebank]

Current output:

error[E0583]: file not found for module `foo`
 --> src/lib.rs:5:5
  |
5 |     mod foo;
  |     ^^^^^^^^
  |
  = help: to create the module `foo`, create file "src/bar/foo.rs" or "src/bar/foo/mod.rs"

[estebank]

@kornelski resurrected your change under #116992