Cannot Load Custom Definition File in Repository

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

Related Issues:

Commencez par reproduire l’exemple importdefinitionfiles lié avec la version de TypeScript signalée, puis comparez le comportement avec typescript@next. Lisez main.ts, messageformat.d.ts et les paramètres de tsconfig.json référencés en retraçant pourquoi la déclaration locale n’est pas trouvée. C’est terminé lorsque la déclaration personnalisée est découverte et que l’import est compilé sans l’erreur de déclaration manquante.