Intralinks to `impl` items defined in another modules are ignored

[orium]

Currently cargo-rdme only explores modules that are mentioned in the documentation intralinks. This means that do not support mentions to Foo::a_method() in the code below, because module a_mod is not explored.

struct Foo {}

mod a_mod {
    impl Foo {
        fn a_method() {}
    }
}

We would like to support this, but we would have to explore all modules, not only the ones explicitly mentioned. For large crates that can carry a big performance impact.