Take into account use statements when resolving stub paths

[aaronbembenek-aws]

Currently, we do not account for use statements when trying to resolve paths appearing in kani::stub attributes. For example, this harness fails to compile:

fn foo() -> bool {
    false
}

fn bar() -> bool {
    true
}

mod my_mod {
    use super::*;

    #[kani::proof]
    #[kani::stub(foo, bar)]
    fn harness() {
        assert!(foo());
    }
}

We are not able to resolve either foo or bar. We should fix this, both to make easier to specify stubs, and to ensure that stubs aren't resolving to the wrong functions/methods.