cmd/go/internal/modload: use atomic.Pointer for Requirements.graph
Change-Id: Ie543e1b1df667cfaf3aafa4be727881461ee8b7d
GitHub-Last-Rev:
ed993dbe2445c4797303138b62f6c7e26050dcd4
GitHub-Pull-Request: golang/go#54888
Reviewed-on: https://go-review.googlesource.com/c/go/+/428716
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>