Improve assist qualified to top when on first segment

[A4-Tacks]

Close #18049

Example

mod std { pub mod fmt { pub trait Debug {} } }
fn main() {
    $0std::fmt::Debug;
    let x: std::fmt::Debug = std::fmt::Debug;
}

Before this PR

use std::fmt;

mod std { pub mod fmt { pub trait Debug {} } }
fn main() {
    fmt::Debug;
    let x: fmt::Debug = fmt::Debug;
}

After this PR

use std::fmt::Debug;

mod std { pub mod fmt { pub trait Debug {} } }
fn main() {
    Debug;
    let x: Debug = Debug;
}