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:

Start by reproducing the linked importdefinitionfiles example with the reported TypeScript version, then compare behavior with typescript@next. Read main.ts, messageformat.d.ts, and the referenced tsconfig.json settings while tracing why the local declaration is not found. Done means the custom declaration is discovered and the import compiles without the missing-declaration error.