Migrate off depswriter.py

### [REQUIRED] Describe your environment

* Operating System version: N/A
* Browser version: N/A
* Firebase SDK version: N/A
* Firebase Product: auth

### [REQUIRED] Describe the problem

In [Closure Library](https://github.com/google/closure-library), we will be removing `depswriter.py` and other Python-based build scripts soon. `depswriter.py` is superceded by [`google-closure-deps`](https://www.npmjs.com/package/google-closure-deps), a similar tool we maintain that is implemented in Node.js.

depswriter.py is used in [this file](https://github.com/firebase/firebase-js-sdk/blob/master/packages/auth/buildtools/generate_test_files.sh), so it will need to be migrated.

I've already created the change and opened a PR here: #4849