refactor: fix/simplify proto gen scripts

[callmehiphop]

Per #541, this PR cleans up the proto scripts and uses mkdirp for Windows support!

[bcoe]

looks great 👍 I'd love to eventually get some Windows testing in our automated testing infrastructure (if it's not in the mix already) using libraries like rimraf and mkdirp are a good initial step to ensure cross-platform compatibility.

[codecov]

Codecov Report

Merging #542 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #542   +/-   ##
=======================================
  Coverage   94.81%   94.81%           
=======================================
  Files          16       16           
  Lines         965      965           
  Branches       86       86           
=======================================
  Hits          915      915           
  Misses         42       42           
  Partials        8        8

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[callmehiphop]

@bcoe we do some Windows testing, but our current CI setup isn't very friendly to non-Googlers, so it's a little bit of a mystery to me to be honest, haha. @jkwlui could probably share some details with us!