This cleans up some of the organization of the python generator. It
will make the extension entry point generation simpler.

It also changes the header guards to use more underscores, which
produces a small diff. Also updates the copyright year in a few
generated files.

Bug: angleproject:2263
Change-Id: I42f061c24a6cfcd8328c56c57eaed9ca6c7bb293
Reviewed-on: https://chromium-review.googlesource.com/846306Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>