proto: move T->*T wrappers from internal/scalar to proto

Usage of these is pervasive in code which works with proto2, and proto2
will be with us for a long, long time to come. Move them to the proto
package.

Change-Id: I1b2e57429fd5a8f107a848a4492d20c27f304bd7
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/185543
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/proto/isinit_test.go b/proto/isinit_test.go
index 1edbfb4..72dd108 100644
--- a/proto/isinit_test.go
+++ b/proto/isinit_test.go
@@ -8,7 +8,6 @@
 	"fmt"
 	"testing"
 
-	"google.golang.org/protobuf/internal/scalar"
 	"google.golang.org/protobuf/proto"
 
 	testpb "google.golang.org/protobuf/internal/testprotos/test"
@@ -32,7 +31,7 @@
 		{
 			&testpb.TestRequiredForeign{
 				RepeatedMessage: []*testpb.TestRequired{
-					{RequiredField: scalar.Int32(1)},
+					{RequiredField: proto.Int32(1)},
 					{},
 				},
 			},