Merge imports fix

[valff]

Fixes #3750

As per rust-lang/rust#60941 (comment), rustc works as intended. This PR solves the issue by avoiding self in path segments of depth past 2.

use self::foo; // it's fine
use foo::{self, bar}; // `foo` can't be a function/macro
// Intentional duplication of `vec` segment, because `vec` can be a function/macro
use alloc::{vec, vec::Vec};

[valff]

cc #3362, rust-lang/rust#63842, rust-lang/rust#60941

[topecongiro]

Thanks!