cmd/go: add the Retract field to 'go help mod edit' definition of the GoMod struct
Fixes #43281
Change-Id: Ife26ca174a8818b56aaea9547976d97978478a5f
GitHub-Last-Rev:
85a3d30001672b371a58d1c8a2092fc9b937af6f
GitHub-Pull-Request: golang/go#43315
Reviewed-on: https://go-review.googlesource.com/c/go/+/279592
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Trust: Jay Conrod <jayconrod@google.com>