cmd/go: recommend 'go get' command to switch from retracted versions

This CL restores a message unintentionally removed in CL 270858.

For #24031

Change-Id: I957c5c59e624df98e72dfff351298bfc5bf9a9e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/272066
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

src/cmd/go/internal/modget/get.go

@@ -1567,11 +1567,20 @@ func (r *resolver) checkPackagesAndRetractions(ctx context.Context, pkgPatterns
 		})
 	}
 	<-r.work.Idle()
+	var retractPath string
 	for _, r := range retractions {
 		if r.err != nil {
 			fmt.Fprintf(os.Stderr, "go: warning: %v\n", r.err)
+			if retractPath == "" {
+				retractPath = r.m.Path
+			} else {
+				retractPath = "<module>"
+			}
 		}
 	}
+	if retractPath != "" {
+		fmt.Fprintf(os.Stderr, "go: run 'go get %s@latest' to switch to the latest unretracted version\n", retractPath)
+	}
 }
 
 // reportChanges logs version changes to os.Stderr.

src/cmd/go/testdata/script/mod_get_retract.txt

@@ -11,6 +11,7 @@ cp go.mod.orig go.mod
 go mod edit -require example.com/retract/self/prev@v1.9.0
 go get -d example.com/retract/self/prev
 stderr '^go: warning: example.com/retract/self/prev@v1.9.0: retracted by module author: self$'
+stderr '^go: run ''go get example.com/retract/self/prev@latest'' to switch to the latest unretracted version$'
 go list -m example.com/retract/self/prev
 stdout '^example.com/retract/self/prev v1.9.0$'