cmd/go: add span for modload.LoadBuildList

This change adds context, and a span to modload.LoadBuildList and
propagates context into modload.BuildList. It's the start
of a run of CLs to add trace spans for module operations.

Updates #38714

Change-Id: I0d58dd394051526338092dc9a5ec29a9e087e4e4
Reviewed-on: https://go-review.googlesource.com/c/go/+/248325


Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>