Poor error guidance for deep imports of packages with @types

[DanielRosenwasser]

yarn init --yes
yarn add @types/acorn@4.0.3

With noImplicitAny:

import * as acorn from "acorn/dist/acorn_loose";

Error:

Could not find a declaration file for module 'acorn/dist/acorn_loose'. 'c:/Users/Daniel/sample/node_modules/acorn/dist/acorn_loose.js' implicitly has an 'any' type.
  Try `npm install @types/acorn` if it exists or add a new declaration (.d.ts) file containing `declare module 'acorn';`

But @types/acorn already has been installed. The true error is that the declaration files haven't been written for this deeper import. I think we can give a better error message here.

[mhegazy]

The suggestion here is to make the error more helpful by checking the existence of the top-level package.