internal/imports: update stdlib index for 1.16

    $ go run mkstdlib.go

Updates golang/go#38706.

Change-Id: I9d5bce3083d2a57eea2ea89c6a99a410ed169f05
Reviewed-on: https://go-review.googlesource.com/c/tools/+/293839
Run-TryBot: Akhil Indurti <aindurti@gmail.com>
Run-TryBot: Heschi Kreinick <heschi@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Trust: Heschi Kreinick <heschi@google.com>
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

internal/imports/mkstdlib.go

@@ -14,7 +14,6 @@ import (
 	"bytes"
 	"fmt"
 	"go/format"
-	exec "golang.org/x/sys/execabs"
 	"io"
 	"io/ioutil"
 	"log"
@@ -23,6 +22,8 @@ import (
 	"regexp"
 	"runtime"
 	"sort"
+
+	exec "golang.org/x/sys/execabs"
 )
 
 func mustOpen(name string) io.Reader {
@@ -66,6 +67,7 @@ func main() {
 		mustOpen(api("go1.13.txt")),
 		mustOpen(api("go1.14.txt")),
 		mustOpen(api("go1.15.txt")),
+		mustOpen(api("go1.16.txt")),
 
 		// The API of the syscall/js package needs to be computed explicitly,
 		// because it's not included in the GOROOT/api/go1.*.txt files at this time.