Cleans up the generator scripts to prepare for listing outputs in the
generated hashes file. Also reorganizes the scripts somewhat to make
them a bit more maintainable.

Bug: angleproject:3227
Change-Id: If40946c2004941d3f6cd51d5521c7db8cc0b52df
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1512052Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>