all: rename tag and flags for legacy support

Rename build tag "proto1_legacy" -> "protolegacy"
to be consistent with the "protoreflect" tag.

Rename flag constant "Proto1Legacy" -> "ProtoLegacy" since
it covers more than simply proto1 legacy features.
For example, it covers alpha-features of proto3 that
were eventually removed from the final proto3 release.

Change-Id: I0f4fcbadd4b5a61c87645e2e5be11d187e59157c
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/189345
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/proto/messageset.go b/proto/messageset.go
index e5d4bd5..14ed199 100644
--- a/proto/messageset.go
+++ b/proto/messageset.go
@@ -25,7 +25,7 @@
 }
 
 func marshalMessageSet(b []byte, m protoreflect.Message, o MarshalOptions) ([]byte, error) {
-	if !flags.Proto1Legacy {
+	if !flags.ProtoLegacy {
 		return b, errors.New("no support for message_set_wire_format")
 	}
 	var err error
@@ -53,7 +53,7 @@
 }
 
 func unmarshalMessageSet(b []byte, m protoreflect.Message, o UnmarshalOptions) error {
-	if !flags.Proto1Legacy {
+	if !flags.ProtoLegacy {
 		return errors.New("no support for message_set_wire_format")
 	}
 	md := m.Descriptor()