Better diagnostics when user forgets to wrap code in `main` function

[frewsxcv]

// some_third_party_crate/lib.rs

mod some_third_party_crate {
    pub fn show() {}
}

//////////////////////////////////////////
//////////////////////////////////////////
//////////////////////////////////////////

// main.rs

some_third_party_crate::show();

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=8fca04bfa35f22c6776578d6c9fd8fbc

We should suggest the user wrap their code in fn main() { ... }

[petrochenkov]

cc #69445

[frewsxcv]

Triage: no change here