encoding/protobuf: implement time type conversions

Adheres to protobuf JSON mapping.

Also fixes a bug where a filed could mask the short
name of the import time packag.

Change-Id: Ib5e71023a4d68492e35da02e997c47f4e09e3dd5
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2722
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/encoding/protobuf/protobuf_test.go b/encoding/protobuf/protobuf_test.go
index d62951d..bd5a905 100644
--- a/encoding/protobuf/protobuf_test.go
+++ b/encoding/protobuf/protobuf_test.go
@@ -120,7 +120,7 @@
 		gotFiles[rel] = f
 	}
 
-	filepath.Walk("testdata/istio.io/api", func(path string, fi os.FileInfo, err error) error {
+	_ = filepath.Walk("testdata/istio.io/api", func(path string, fi os.FileInfo, err error) error {
 		if err != nil || fi.IsDir() || !strings.HasSuffix(path, ".cue") {
 			return err
 		}