swarming: Use is_valid_package_name_template for external service defaults.
swarming: Use is_valid_package_name_template for external service defaults.

Instead of just 'is_valid_package_name'. Otherwise, CIPD doesn't seem
to like the magic platform var:
Config validation: cipd client_package is invalid "infra/tools/cipd/${platform}"

Bug: 812428
Change-Id: I1442543722a8b386c41c72ec5dcaee8c22bc90e3
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-py/+/2068941
Reviewed-by: Takuto Ikuta <71927f930fca81015e0f496c65cd126ab1a692e3@chromium.org>
Commit-Queue: Ben Pastene <e214258550f76fa23ac7c1d953424f3f468f5c6b@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py