security_scanning: Add external/github.com prefix rule

Replace the individual repo rules under external/github.com with a
single repo_prefix rule since all of these are open source mirrors.

BUG=b:322042105
TEST=cros lint and cros format

Change-Id: I6f2d97fd1c9e74e28715e16f54326cea76cd8cdb
Reviewed-on: https://chromium-review.googlesource.com/c/All-Projects/+/5260346
Reviewed-by: Mike Frysinger <vapier@chromium.org>