Flow reports errors about missing modules

Nobody has claimed this yet.

**mapbox-gl-js version**: 0.48

### Steps to Trigger Behavior

1. Update mapbox-gl and Flow to latest versions
2. Add `node_modules/mapbox-gl/flow-typed` to [libs] section in .flowconfig
3. Run Flow analysis

### Link to Demonstration

The problem can be seen in repo https://github.com/mikkom/onnikka by running `yarn && yarn flow`.

### Expected Behavior

No Flow errors (at least from within mapbox-gl).

### Actual Behavior

Flow reports two errors where it cannot resolve a module.

```
Error ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ node_modules/mapbox-gl/src/util/window.js:5:16

Cannot resolve module gl.

2│
3│ import jsdom from 'jsdom';
4│
5│ import gl from 'gl';
6│ import sinon from 'sinon';
7│ import { extend } from './util';
8│

Error ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ node_modules/mapbox-gl/src/util/window.js:6:19

Cannot resolve module sinon.

3│ import jsdom from 'jsdom';
4│
5│ import gl from 'gl';
6│ import sinon from 'sinon';
7│ import { extend } from './util';
8│
9│ import type {Window} from '../types/window';
```

Manually adding gl and sinon as dependencies in the project removes both errors but I think that should not be necessary?

Reproduce the report with `yarn && yarn flow` in the linked onnikka repository, then inspect `src/util/window.js`, especially its `gl` and `sinon` imports, together with the `flow-typed` setup. Done means Flow no longer reports module-resolution errors from mapbox-gl without manually adding those dependencies.

Written by the indexing model from the issue text.