TypeScript Version: 2.8.1

Search Terms: Definition File, Ambient type
Detail
I am importing a JavaScript library that doesn't have any definition file. I want to create a custom definition file that I place in my repository. I want TypeScript to read this definition file.

I have tried many combination of tsconfig.json (typeRoot, include, path, baseUrl, etc.) Without being successful. I also moved the definition folder under src, and renamed the definition file to index.d.ts (inside a folder with the name of the library) without any success.

Code
Small complete repro here: https://github.com/MrDesjardins/importdefinitionfiles

// main.ts:
import MessageFormat from "messageformat"; // Doesn't have type
// messageformat.d.ts:
declare module messageformat {
    export type Msg = (params: {}) => string;
    export interface MessageFormat {
        new(message: string): any;
        compile: (messageSource: string) => Msg;
    }
}

Expected behavior: TypeScript to find the definition file and to use it without having a compilation error.

Actual behavior: Error message

Could not find a declaration file for module 'messageformat'. '/Users/pdesjardins/code/perso/importdefinitionfiles/node_modules/messageformat/lib/messageformat.js' implicitly has an 'any' type.
  Try `npm install @types/messageformat` if it exists or add a new declaration (.d.ts) file containing `declare module 'messageformat';`

1 import MessageFormat from "messageformat";

Playground Link: https://github.com/MrDesjardins/importdefinitionfiles

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