$ export_targets.py <out_dir> <targets...>

Flattens all library targets under <targets...> and checks that all
includes in 'sources' are known to the build.

Firefox can use this as the first half of our ANGLE vendoring process.

Included additions to src/libGLESv2.gni were found with:
$ export_targets.py out //:libEGL

Eventually, it would be nice to have this as a presubmit check.

Bug: angleproject:3403
Change-Id: I0f455e8c37c4b0a9e17e9f843843422b6a32f588
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1584652
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>