cmd/go: add -retract and -dropretract flags to 'go mod edit'

cmd/go: add -retract and -dropretract flags to 'go mod edit'

'go mod edit' can now add and remove 'retract' directives from go.mod
files.

Also, retractions are now included in the 'go mod edit -json' output.

For #24031

Change-Id: Ife7915e259fa508626d6ec5f786b5c860b489599
Reviewed-on: https://go-review.googlesource.com/c/go/+/228381
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>