Find all references for ambient module

``` // declarations.d.ts declare module "x" { export const x: number; } // index.ts import {x} from "x"; ``` Goto-declaration for "x" in index.ts works, but find-all-references for "x" in declarations.d.ts finds only itself. In this case one can simply look through each of export in the module's body and find all of their references; but for shorthand ambient modules (`declare module "x";`) that won't work, as there is no module body.