cmd/coordinator: create separate spanlog package

This moves main.spanLogger and main.eventSpan to spanlog.Logger and
spanlog.Span. This change is necessary to start pulling build and
benchmark code out of coordinator. The interfaces cannot simply be
copied because Logger contains a function whose return type is
Span, so the interface must be defined in exactly one place.

Updates golang/go#19871

Change-Id: I0a48192e6a5c8f5d0445f4f3d3cce8d91c90f8d3
Reviewed-by: Brad Fitzpatrick <>
6 files changed