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/internal/encoding/messageset/messageset.go b/internal/encoding/messageset/messageset.go
index 7d05dee..b1c6db5 100644
--- a/internal/encoding/messageset/messageset.go
+++ b/internal/encoding/messageset/messageset.go
@@ -65,7 +65,7 @@
 	if err != nil {
 		return nil, err
 	}
-	if !IsMessageSetExtension(xt) {
+	if !IsMessageSetExtension(xt.Descriptor()) {
 		return nil, preg.NotFound
 	}
 	return xt, nil