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Split the Perl script into 2 and shard the actual checking, so we can
run it in parallel. On bots, this reduces the time needed to complete
"check licenses" step from 3-4 mins to 40 seconds!

I'm planning to rewrite the scanner in Python soon, but I see that this
change can already help running trybots faster.

BUG=416496

Review URL: https://codereview.chromium.org/589143002

Cr-Commit-Position: refs/heads/master@{#296207}