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/encoding/protojson/decode.go b/encoding/protojson/decode.go
index 1d22d3a..5e32d8b 100644
--- a/encoding/protojson/decode.go
+++ b/encoding/protojson/decode.go
@@ -139,7 +139,7 @@
 // unmarshalFields unmarshals the fields into the given protoreflect.Message.
 func (o UnmarshalOptions) unmarshalFields(m pref.Message, skipTypeURL bool) error {
 	messageDesc := m.Descriptor()
-	if !flags.Proto1Legacy && messageset.IsMessageSet(messageDesc) {
+	if !flags.ProtoLegacy && messageset.IsMessageSet(messageDesc) {
 		return errors.New("no support for proto1 MessageSets")
 	}
 
diff --git a/encoding/protojson/decode_test.go b/encoding/protojson/decode_test.go
index 9c4f93e..c343104 100644
--- a/encoding/protojson/decode_test.go
+++ b/encoding/protojson/decode_test.go
@@ -1334,7 +1334,7 @@
 			})
 			return m
 		}(),
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc:         "not real MessageSet 1",
 		inputMessage: &pb2.FakeMessageSet{},
@@ -1350,7 +1350,7 @@
 			})
 			return m
 		}(),
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc:         "not real MessageSet 2",
 		inputMessage: &pb2.FakeMessageSet{},
@@ -1360,7 +1360,7 @@
   }
 }`,
 		wantErr: true,
-		skip:    !flags.Proto1Legacy,
+		skip:    !flags.ProtoLegacy,
 	}, {
 		desc:         "not real MessageSet 3",
 		inputMessage: &pb2.MessageSet{},
@@ -1376,7 +1376,7 @@
 			})
 			return m
 		}(),
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc:         "Empty",
 		inputMessage: &emptypb.Empty{},
@@ -2457,13 +2457,13 @@
 			m.SetWeakMessage1(&weakpb.WeakImportMessage1{A: proto.Int32(1)})
 			return m
 		}(),
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc:         "weak fields; unknown field",
 		inputMessage: &testpb.TestWeak{},
 		inputText:    `{"weak_message1":{"a":1}, "weak_message2":{"a":1}}`,
 		wantErr:      true, // weak_message2 is unknown since the package containing it is not imported
-		skip:         !flags.Proto1Legacy,
+		skip:         !flags.ProtoLegacy,
 	}}
 
 	for _, tt := range tests {
diff --git a/encoding/protojson/encode.go b/encoding/protojson/encode.go
index 9b81eca..839c7d1 100644
--- a/encoding/protojson/encode.go
+++ b/encoding/protojson/encode.go
@@ -88,7 +88,7 @@
 // marshalFields marshals the fields in the given protoreflect.Message.
 func (o MarshalOptions) marshalFields(m pref.Message) error {
 	messageDesc := m.Descriptor()
-	if !flags.Proto1Legacy && messageset.IsMessageSet(messageDesc) {
+	if !flags.ProtoLegacy && messageset.IsMessageSet(messageDesc) {
 		return errors.New("no support for proto1 MessageSets")
 	}
 
diff --git a/encoding/protojson/encode_test.go b/encoding/protojson/encode_test.go
index 5cacf9e..9d7a8ae 100644
--- a/encoding/protojson/encode_test.go
+++ b/encoding/protojson/encode_test.go
@@ -1034,7 +1034,7 @@
     "optString": "not a messageset extension"
   }
 }`,
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc: "not real MessageSet 1",
 		input: func() proto.Message {
@@ -1049,7 +1049,7 @@
     "optString": "not a messageset extension"
   }
 }`,
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc: "not real MessageSet 2",
 		input: func() proto.Message {
@@ -1064,7 +1064,7 @@
     "optString": "another not a messageset extension"
   }
 }`,
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc:  "BoolValue empty",
 		input: &wrapperspb.BoolValue{},
diff --git a/encoding/prototext/decode.go b/encoding/prototext/decode.go
index 06388c8..7207dde 100644
--- a/encoding/prototext/decode.go
+++ b/encoding/prototext/decode.go
@@ -76,7 +76,7 @@
 // unmarshalMessage unmarshals a [][2]text.Value message into the given protoreflect.Message.
 func (o UnmarshalOptions) unmarshalMessage(tmsg [][2]text.Value, m pref.Message) error {
 	messageDesc := m.Descriptor()
-	if !flags.Proto1Legacy && messageset.IsMessageSet(messageDesc) {
+	if !flags.ProtoLegacy && messageset.IsMessageSet(messageDesc) {
 		return errors.New("no support for proto1 MessageSets")
 	}
 
diff --git a/encoding/prototext/decode_test.go b/encoding/prototext/decode_test.go
index 588d9ee..636e21b 100644
--- a/encoding/prototext/decode_test.go
+++ b/encoding/prototext/decode_test.go
@@ -1310,7 +1310,7 @@
 			})
 			return m
 		}(),
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc:         "not real MessageSet 1",
 		inputMessage: &pb2.FakeMessageSet{},
@@ -1326,7 +1326,7 @@
 			})
 			return m
 		}(),
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc:         "not real MessageSet 2",
 		inputMessage: &pb2.FakeMessageSet{},
@@ -1336,7 +1336,7 @@
 }
 `,
 		wantErr: true,
-		skip:    !flags.Proto1Legacy,
+		skip:    !flags.ProtoLegacy,
 	}, {
 		desc:         "not real MessageSet 3",
 		inputMessage: &pb2.MessageSet{},
@@ -1351,7 +1351,7 @@
 			})
 			return m
 		}(),
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc:         "Any not expanded",
 		inputMessage: &anypb.Any{},
@@ -1501,13 +1501,13 @@
 			m.SetWeakMessage1(&weakpb.WeakImportMessage1{A: proto.Int32(1)})
 			return m
 		}(),
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc:         "weak fields; unknown field",
 		inputMessage: &testpb.TestWeak{},
 		inputText:    `weak_message1:{a:1} weak_message2:{a:1}`,
 		wantErr:      true, // weak_message2 is unknown since the package containing it is not imported
-		skip:         !flags.Proto1Legacy,
+		skip:         !flags.ProtoLegacy,
 	}}
 
 	for _, tt := range tests {
diff --git a/encoding/prototext/encode.go b/encoding/prototext/encode.go
index 5d8cd46..b732f02 100644
--- a/encoding/prototext/encode.go
+++ b/encoding/prototext/encode.go
@@ -75,7 +75,7 @@
 // marshalMessage converts a protoreflect.Message to a text.Value.
 func (o MarshalOptions) marshalMessage(m pref.Message) (text.Value, error) {
 	messageDesc := m.Descriptor()
-	if !flags.Proto1Legacy && messageset.IsMessageSet(messageDesc) {
+	if !flags.ProtoLegacy && messageset.IsMessageSet(messageDesc) {
 		return text.Value{}, errors.New("no support for proto1 MessageSets")
 	}
 
diff --git a/encoding/prototext/encode_test.go b/encoding/prototext/encode_test.go
index 493a229..0c480e6 100644
--- a/encoding/prototext/encode_test.go
+++ b/encoding/prototext/encode_test.go
@@ -1078,7 +1078,7 @@
   opt_string: "not a messageset extension"
 }
 `,
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc: "not real MessageSet 1",
 		input: func() proto.Message {
@@ -1092,7 +1092,7 @@
   opt_string: "not a messageset extension"
 }
 `,
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc: "not real MessageSet 2",
 		input: func() proto.Message {
@@ -1106,7 +1106,7 @@
   opt_string: "another not a messageset extension"
 }
 `,
-		skip: !flags.Proto1Legacy,
+		skip: !flags.ProtoLegacy,
 	}, {
 		desc: "Any not expanded",
 		mo: prototext.MarshalOptions{