fix: treat {self} in braced use as parent path

[quantumshiro]

issure number #1113
please check my codes.
It should be working without any particular problems.

shiro@Shiro:~/workspace/fe$ cargo run -q -p fe -- check test_self_import.fe
shiro@Shiro:~/workspace/fe$ cat test_self_import.fe
use core::option::Option::{self, Some, None}

fn test() -> Option<u32> {
    Some(42)
}

[sbillig]

Perfect, thank you @quantumshiro