cmd/fiximports,cmd/present,cmd/stringer: update links to pkg.go.dev

These commands link to self-documentation hosted on godoc.org. Given
that this traffic will eventually be redirected to pkg.go.dev
(https://blog.golang.org/pkg.go.dev-2020), we should proactively update
them to pkg.go.dev.

For golang/go#42251

Change-Id: Iff1e4c6958d4e1ddff98150fae6b811e7764816a
Reviewed-on: https://go-review.googlesource.com/c/tools/+/264999
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Julie Qiu <julie@golang.org>
Trust: Robert Findley <rfindley@google.com>

cmd/fiximports/main.go

@@ -114,7 +114,7 @@ The package... arguments specify a list of packages
 in the style of the go tool; see "go help packages".
 Hint: use "all" or "..." to match the entire workspace.
 
-For details, see http://godoc.org/golang.org/x/tools/cmd/fiximports.
+For details, see https://pkg.go.dev/golang.org/x/tools/cmd/fiximports
 
 Flags:
   -n:	       dry run: show changes, but don't apply them

cmd/present/doc.go

@@ -47,6 +47,6 @@ the generated output. The supported formats are:
 	.article      // article format, such as a blog post
 
 The present file format is documented by the present package:
-http://godoc.org/golang.org/x/tools/present
+https://pkg.go.dev/golang.org/x/tools/present
 */
 package main

cmd/stringer/stringer.go

@@ -98,7 +98,7 @@ func Usage() {
 	fmt.Fprintf(os.Stderr, "\tstringer [flags] -type T [directory]\n")
 	fmt.Fprintf(os.Stderr, "\tstringer [flags] -type T files... # Must be a single package\n")
 	fmt.Fprintf(os.Stderr, "For more information, see:\n")
-	fmt.Fprintf(os.Stderr, "\thttp://godoc.org/golang.org/x/tools/cmd/stringer\n")
+	fmt.Fprintf(os.Stderr, "\thttps://pkg.go.dev/golang.org/x/tools/cmd/stringer\n")
 	fmt.Fprintf(os.Stderr, "Flags:\n")
 	flag.PrintDefaults()
 }