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/encoding/text/text_test.go b/internal/encoding/text/text_test.go
index 0e40fa8..64771f3 100644
--- a/internal/encoding/text/text_test.go
+++ b/internal/encoding/text/text_test.go
@@ -615,7 +615,7 @@
 	}, {
 		in: `nums: [0xbeefbeef,0xbeefbeefbeefbeef]`,
 		wantVal: V(Msg{{ID("nums"), func() Value {
-			if flags.Proto1Legacy {
+			if flags.ProtoLegacy {
 				return V(Lst{V(int32(-1091584273)), V(int64(-4688318750159552785))})
 			} else {
 				return V(Lst{V(uint32(0xbeefbeef)), V(uint64(0xbeefbeefbeefbeef))})