Auto Import structure breaks with `"rust-analyzer.imports.prefix": "plain"`

I do not believe this is a regression, but it is exacerbated by #17277.

rust-analyzer version: 0.3.1975-standalone (71a816a90 2024-05-26)

rustc version: 1.78.0 (9b00956e5 2024-04-29)

editor or extension: VSCode

relevant settings: rust-analyzer.imports.prefix

to reproduce:

1. Create a new project with serde (including feature derive) as a dependency; any external crate would work, but I chose serde for this report.
2. Create the following src directory

├── lib.rs
├── foo.rs

and the following files.

// lib.rs
use serde::Serialize;

mod foo;

#[derive(Serialize)]
struct Example;

impl Example {
    fn a() {
    }
}

// foo.rs
pub fn b() {
}

It is important that the module is named foo, so it is alphabetically before serde.
3. Through UI or JSON, change the setting rust-analyzer.imports.prefix to self.
4. In the body of Example::a(), type b and use auto-import. Observe that self::foo::b is correctly imported in a group below serde::Serialize. Remove the function call and import, so it once again resembles Step 2.
5. Change rust-analyzer.imports.prefix to plain (the default).
6. In the body of Example::a(), type b and use auto-import. Observe that foo::c is incorrectly imported. It should be below serde::Serialize, as it is a current module import, but it is instead above serde::Serialize in the same group, as if it was an external crate.

Reproduce the behavior using the minimal project in src/lib.rs and src/foo.rs, with serde and the rust-analyzer.imports.prefix setting changed between self and plain. Trace the auto-import behavior for b inside Example::a(); done means plain places the current-module import below serde::Serialize rather than grouping it as an external import.