internal/proto: move location of cue/cue.proto

This makes cuelang.org/cue/encoding/protobuf (git)
the import path and cue/cue.proto the canonical
import name.

Add google files for import, even though the types
are mapped, for better validation.

Change-Id: I8f31102baf362608c50c26f85e810b87321aab94
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2342
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/internal/protobuf/parse.go b/internal/protobuf/parse.go
index 712c42e..0753199 100644
--- a/internal/protobuf/parse.go
+++ b/internal/protobuf/parse.go
@@ -244,7 +244,7 @@
 }
 
 func (p *protoConverter) doImport(v *proto.Import) {
-	if v.Filename == "cuelang/cue.proto" {
+	if v.Filename == "cue/cue.proto" {
 		return
 	}
 
@@ -259,10 +259,7 @@
 		break
 	}
 
-	if filename == "" {
-		p.mustBuiltinPackage(v.Position, v.Filename)
-		return
-	}
+	p.mapBuiltinPackage(v.Position, v.Filename, filename == "")
 
 	imp, err := p.state.parse(filename, nil)
 	if err != nil {
@@ -356,6 +353,9 @@
 	case *proto.Import:
 		// already handled.
 
+	case *proto.Extensions:
+		// no need to handle
+
 	default:
 		failf(scanner.Position{}, "unsupported type %T", x)
 	}
@@ -440,6 +440,9 @@
 	case *proto.Oneof:
 		p.oneOf(x)
 
+	case *proto.Extensions:
+		// no need to handle
+
 	default:
 		failf(scanner.Position{}, "unsupported type %T", v)
 	}