cmd/protoc-gen-go: generate descriptor proto using v2 textpb

Changes:
* Modify protoc-gen-go to use a helper from internal/impl
to print the message as text.
* Add a helper function to internal/impl that calls v2 textpb.
* Modify encoding/textpb to avoid depending on descriptor proto,
which would cause an import cycle.
* Modify internal/fileinit to populate a pseudo-internal
method on MessageDescriptor to check whether the message should
use the message set wire format. We avoid adding this to the
main MessageDescriptor interface since message sets are a
deprecated proto1 feature that we should avoid promoting.

Change-Id: Ibaf79a563af695756f11ddc4db69b38e25a8f1a7
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/168439
Reviewed-by: Herbie Ong <herbie@google.com>
diff --git a/internal/impl/export.go b/internal/impl/export.go
index 6e54939..12ce51b 100644
--- a/internal/impl/export.go
+++ b/internal/impl/export.go
@@ -7,6 +7,7 @@
 import (
 	"strconv"
 
+	"github.com/golang/protobuf/v2/encoding/textpb"
 	ptype "github.com/golang/protobuf/v2/internal/prototype"
 	pref "github.com/golang/protobuf/v2/reflect/protoreflect"
 )
@@ -86,3 +87,10 @@
 	}
 	return legacyWrapper.ExtensionTypeOf(d, t)
 }
+
+// MessageStringOf returns the message value as a string,
+// which is the message serialized in the protobuf text format.
+func (Export) MessageStringOf(m pref.ProtoMessage) string {
+	b, _ := textpb.MarshalOptions{Compact: true}.Marshal(m)
+	return string(b)
+}