Item resolver incorrectly finds instance functions when referencing local names

[udoprog]

The following program fails to run:

struct Test;

fn call(a, b) {
    a + b
}

impl Test {
    fn call(self) {
        call(1, 2)
    }
}

pub fn main() {
    let test = Test;
    assert_eq!(test.call(), 3);
}

This is because the call(1, 2) incorrectly resolves to Test::call.