Absolute paths are not correct for re-exported items

[mitaa]

(extracted from #1920)

@nikomatsakis

Another related issue here is that we have no way to take into account re-exports -- see the reference to core, for example, and not std.
(Handling re-exports may require some sort of annotations though.)

fn assert_clone<T>() where T : Clone { }

fn main() {
    assert_clone::<std::sync::atomic::AtomicBool>();
}

<anon>:4:5: 4:50 error: the trait `core::clone::Clone` is not implemented for the type `core::sync::atomic::AtomicBool` [E0277]
<anon>:4     assert_clone::<std::sync::atomic::AtomicBool>();
             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This should show std::clone::Clone and std::sync::atomic::AtomicBool since core isn't even linked into or reachable from the local crate.

[apasel422]

See #23224, #23355, #26635, #27165.