x/net/icmp: new package

This CL makes icmp an external package.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/182370043

internal/icmp/example_test.go → icmp/example_test.go

@@ -9,8 +9,8 @@ import (
 	"net"
 	"os"
 
+	"golang.org/x/net/icmp"
 	"golang.org/x/net/internal/iana"
-	"golang.org/x/net/internal/icmp"
 	"golang.org/x/net/ipv6"
 )
 

internal/icmp/message_test.go → icmp/message_test.go

@@ -9,8 +9,8 @@ import (
 	"reflect"
 	"testing"
 
+	"golang.org/x/net/icmp"
 	"golang.org/x/net/internal/iana"
-	"golang.org/x/net/internal/icmp"
 	"golang.org/x/net/ipv4"
 	"golang.org/x/net/ipv6"
 )

internal/icmp/ping_test.go → icmp/ping_test.go

@@ -11,8 +11,8 @@ import (
 	"runtime"
 	"testing"
 
+	"golang.org/x/net/icmp"
 	"golang.org/x/net/internal/iana"
-	"golang.org/x/net/internal/icmp"
 	"golang.org/x/net/ipv4"
 	"golang.org/x/net/ipv6"
 )