[cipd] Cleanup ExtractFiles implementation a bit.
[cipd] Cleanup ExtractFiles implementation a bit.

It had some confusing code (still does, but less so).

R=40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org
BUG=1217435

Change-Id: Ia19e52ce1a88c0d3d1ad8373eb19b8aaa6225ec1
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2970707
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go