Detect when unresolved import is caused by  item that's `cfg`ed away

When having an annotated import like `#[cfg(test)] use foo::bar;` and elsewhere in the same scope we have `use self::bar::baz;` we currently get an error about unresolved imports when building regularly. We should point out that the import would be valid if it wasn't restricted to `test` runs and suggest to either remove the other imports annotation or adding one to the current import.