all: refactor extensions, add proto.GetExtension etc.

Change protoiface.ExtensionDescV1 to implement protoreflect.ExtensionType.

ExtensionDescV1's Name field conflicts with the Descriptor Name method,
so change the protoreflect.{Message,Enum,Extension}Type types to no
longer implement the corresponding Descriptor interface. This also leads
to a clearer distinction between the two types.

Introduce a protoreflect.ExtensionTypeDescriptor type which bridges
between ExtensionType and ExtensionDescriptor.

Add extension accessor functions to the proto package:
proto.{Has,Clear,Get,Set}Extension. These functions take a
protoreflect.ExtensionType parameter, which allows writing the
same function call using either the old or new API:

  proto.GetExtension(message, somepb.E_ExtensionFoo)

Fixes golang/protobuf#908

Change-Id: Ibc65d12a46666297849114fd3aefbc4a597d9f08
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/189199
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/testing/prototest/prototest_test.go b/testing/prototest/prototest_test.go
index a95ac0b..00df3a6 100644
--- a/testing/prototest/prototest_test.go
+++ b/testing/prototest/prototest_test.go
@@ -10,9 +10,12 @@
 
 	"google.golang.org/protobuf/internal/flags"
 	"google.golang.org/protobuf/proto"
+	"google.golang.org/protobuf/runtime/protoimpl"
 	"google.golang.org/protobuf/testing/prototest"
 
 	irregularpb "google.golang.org/protobuf/internal/testprotos/irregular"
+	legacypb "google.golang.org/protobuf/internal/testprotos/legacy"
+	legacy1pb "google.golang.org/protobuf/internal/testprotos/legacy/proto2.v0.0.0-20160225-2fc053c5"
 	testpb "google.golang.org/protobuf/internal/testprotos/test"
 	_ "google.golang.org/protobuf/internal/testprotos/test/weak1"
 	_ "google.golang.org/protobuf/internal/testprotos/test/weak2"
@@ -26,6 +29,8 @@
 		(*testpb.TestRequired)(nil),
 		(*irregularpb.Message)(nil),
 		(*testpb.TestAllExtensions)(nil),
+		(*legacypb.Legacy)(nil),
+		protoimpl.X.MessageOf((*legacy1pb.Message)(nil)).Interface(),
 	}
 	if flags.Proto1Legacy {
 		ms = append(ms, (*testpb.TestWeak)(nil))