[release-branch.go1.13] cmd/go/internal/modget: synchronize writes to modOnly map in runGet

Updates #35317
Fixes #35318

Change-Id: Id858a25dc16a1bbff1802d25bcd4aca31c1133bc
Reviewed-on: https://go-review.googlesource.com/c/go/+/205067


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
(cherry picked from commit 7e71c9c3)
Reviewed-on: https://go-review.googlesource.com/c/go/+/205517