cmd/go: add -retracted flag to 'go list'

cmd/go: add -retracted flag to 'go list'

The -retracted flag causes 'go list' to load information about
retracted module module versions.

When -retracted is used with -f or -json, the Retracted field is set
to a string containing the reason for the retraction on retracted
module versions. The string is based on comments on the retract
directive. This field is also populated when the -u flag is used.

When -retracted is used with -versions, retracted versions are shown.
Normally, they are omitted.

For #24031

Change-Id: Ic13d516eddffb1b8404e21034f78cecc9896d1b8
Reviewed-on: https://go-review.googlesource.com/c/go/+/228382
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>