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/flags/proto_legacy_enable.go b/internal/flags/proto_legacy_enable.go
new file mode 100644
index 0000000..dbd2cd5
--- /dev/null
+++ b/internal/flags/proto_legacy_enable.go
@@ -0,0 +1,9 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE.md file.
+
+// +build protolegacy
+
+package flags
+
+const protoLegacy = true