Rollup merge of #147805 - LorrensP-2158466:extern-mod-disamb, r=petrochenkov

use module_child index as disambiguator for external items

When defining the items of an external module, if that item is an underscore we use it's index as the disambiguator. This is needed for parallel import resolution, which is being worked on in rust-lang/rust#145108.

r? `@petrochenkov`

Author: matthiaskrgr