Improve E0432

[dashed]

For the following error code: https://doc.rust-lang.org/error-index.html#E0432

I think it's useful to suggest that the following statement may be needed for main.rs:

extern crate <name>;

[durka]

It actually does say that in the error message:

expr.rs:5:6: 5:15 error: unresolved import `something::Foo`. Maybe a missing `extern crate something`? [E0432]
expr.rs:5 {use something::Foo;}

But not on the error explanations page.