encoding/protobuf: don't use disallowed "__" prefix

Change-Id: If143743b952f993cb97c3493361610c7f10a5d2f
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/3580
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/encoding/protobuf/testdata/attributes.proto.out.cue b/encoding/protobuf/testdata/attributes.proto.out.cue
index 4ac3479..936a6db 100644
--- a/encoding/protobuf/testdata/attributes.proto.out.cue
+++ b/encoding/protobuf/testdata/attributes.proto.out.cue
@@ -128,17 +128,17 @@
 
 	// Holds attributes of type TIMESTAMP
 	time?: {
-		<_>: __time.Time
+		<_>: _time_.Time
 	} @protobuf(6,type=map<sint32,google.protobuf.Timestamp>,"(gogoproto.nullable)=false","(gogoproto.stdtime)")
 
 	// Holds attributes of type DURATION
 	durations?: {
-		<_>: __time.Duration
+		<_>: _time_.Duration
 	} @protobuf(7,type=map<sint32,google.protobuf.Duration>,"(gogoproto.nullable)=false","(gogoproto.stdduration)")
 
 	// Holds attributes of type BYTES
 	bytes?: {
-		<_>: __bytes
+		<_>: _bytes_
 	} @protobuf(8,type=map<sint32,bytes>)
 
 	// Holds attributes of type STRING_MAP
@@ -146,8 +146,8 @@
 		<_>: StringMap
 	} @protobuf(9,type=map<sint32,StringMap>,string_maps,"(gogoproto.nullable)=false")
 }
-__time = time
-__bytes = bytes
+_time_ = time
+_bytes_ = bytes
 
 // A map of string to string. The keys and values in this map are dictionary
 // indices (see the [Attributes][istio.mixer.v1.CompressedAttributes] message for an explanation)