runtime: use internal/trace/tracev2 definitions

runtime: use internal/trace/tracev2 definitions

This change deduplicates trace wire format definitions between the
runtime and the trace parser by making the internal/trace/tracev2
package the source of truth.

Change-Id: Ia0721d3484a80417e40ac473ec32870bee73df09
Reviewed-on: https://go-review.googlesource.com/c/go/+/644221
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>