Support doc links to associated functions on other structs

[lf-]

struct F;
impl F {
    // expect to go here but it fails instead
    fn linkme() {}
}

/// See [`F::linkme`]
//           ^-- go to definition here
fn f1() {}

If I do the above go to definition, it doesn't find a target. rustdoc appears to compile it fine.

The example from #9617 linking to other things on the same top level item does work, it seems to just be linking to things from other top level items that's a trouble here.

Version: 5983d37

[fsktom]

The same thing still happens in 5342f47 (v0.3.1123)

E.g. with

struct Foo {}
impl Foo {
    /// [Foo::baz()] - it links correctly in `cargo doc` but not in VSC!
    fn bar(self) {}

    /// [buzz] - linked properly in VSC and `cargo doc`
    fn baz(self) {}
}

fn buzz() {}

only buzz() is linked properly in VSC

while in the rustdoc generated docs both are linked correctly