GN: get echo-headers sources via exec_script

exec_script runs every time gn does, which is explicitly on every one of our
bot runs.  That should be enough to obviate the .git/logs/HEAD hack.

Easiest way to do this was to swap around find.py's argument order to allow
multiple search directories.  This is the root of all the .gyp changes.

This moves the blacklist into BUILD.gn, which I think is nice.
It expands it a little as we're now searching recursively, into include/gpu/vk
which we can't include safely without the Vulkan SDK.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2205903004

Review-Url: https://codereview.chromium.org/2205903004

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