`"rust-analyzer.imports.prefix": "plain"` is ignored when importing items defined in subdirectories

[homchom]

rust-analyzer version: 0.3.1958-standalone (5bf2f85 2024-05-09)

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 the following src directory

├── lib.rs
├── foo.rs
├── foo
│   ├── bar.rs

and the following files:

// lib.rs
mod foo;

pub fn a() {
}

// foo.rs
mod bar;

pub fn b() {
}

// bar.rs
pub fn c() {
}

2. Through UI or JSON, change the setting rust-analyzer.imports.prefix to plain (the default).
3. In the body of a(), type b and use auto-import. Observe that foo::b is correctly imported.
4. In the body of b(), type c and use auto-import. Observe that self::bar::c is incorrectly imported; it should be bar::c.