Migrate off depswriter.py

[kjin]

[REQUIRED] Describe your environment

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[REQUIRED] Describe the problem

In Closure Library, we will be removing depswriter.py and other Python-based build scripts soon. depswriter.py is superceded by google-closure-deps, a similar tool we maintain that is implemented in Node.js.

depswriter.py is used in this file, so it will need to be migrated.

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

[weixifan]

Hi, I've created b/188396942 to track this.