Typescript declare javascript external modules

So, I'm new to TypeScript and I wanted to convert a really small NodeJS project I was doing in plain JavaScript to TypeScript. In that project, I use some npm modules that have a definition under the @types scope, but others don't.

So, I figured I'd create these declarations for myself in my project, without having to change those third-party modules. However, no matter where I put the .d.ts file, TypeScript does not seem to recognize it.

src/typings.d.ts

import stream = require( 'stream' );
import http = require( 'http' );

declare module "flatten" {
    function flatten ( ...arrays : any[] ) : any[];

    export = flatten;
}

declare module "got" {
    //...
}

And then when I try to use the module flatten, for instance:

src/manager.ts

import flatten = require( 'flatten' );
//...

I get the error:

src/manager.ts(1,27): error TS2307: Cannot find module 'flatten'.

My tsconfig.json looks like this:

{
    "compilerOptions": {
        "target": "es6",
        "module": "commonjs",
        "noImplicitAny": true,
        "sourceMap": true,
        "outDir": "lib/" 
    },
    "include": [
        "src/**/*"
    ],
    "exclude": [
        "node_modules",
        "**/*.spec.ts"
    ]
}