Use google-closure-deps instead of depswriter.py.

[kjin]

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 depswriter.py is superceded by google-closure-deps, a similar tool we maintain that is implemented in Node.js.

Here, I've substituted the depswriter.py command with a corresponding command from google-closure-deps; the CLI flags are similar. The difference in output is minimal (see below).

Explanation of "extra" flags:

• --closure-path="node_modules/google-closure-library/closure/goog" Path to Closure (now required)
• --file="node_modules/google-closure-library/closure/goog/deps.js" Use the deps.js file bundled with Closure as part of deps calculation
• --exclude=... Not strictly necessary, but prevents these files from being written to deps.js as trivial, 0-dependency entries

For reference, the diff of the resulting deps.js from running npm run generate-test-files (with lines sorted and formatted for diff clarity) is here.

[bojeil-google]

Thanks for the fix @kjin. While, testing your change, I discovered some broken tests unrelated to this PR. I will merge this and follow up with the fixes.