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/internal/impl/codec_messageset.go b/internal/impl/codec_messageset.go
index 073c4dc..d5fb444 100644
--- a/internal/impl/codec_messageset.go
+++ b/internal/impl/codec_messageset.go
@@ -45,7 +45,7 @@
 }
 
 func marshalMessageSet(mi *MessageInfo, b []byte, p pointer, wiretag uint64, opts marshalOptions) ([]byte, error) {
-	if !flags.Proto1Legacy {
+	if !flags.ProtoLegacy {
 		return b, errors.New("no support for message_set_wire_format")
 	}
 	ext := *p.Extensions()
@@ -97,7 +97,7 @@
 }
 
 func unmarshalMessageSet(mi *MessageInfo, b []byte, p pointer, wtyp wire.Type, opts unmarshalOptions) (int, error) {
-	if !flags.Proto1Legacy {
+	if !flags.ProtoLegacy {
 		return 0, errors.New("no support for message_set_wire_format")
 	}
 	if wtyp != wire.StartGroupType {