Fix checkCalledFromInit

This function checks if it is called from init() by looking into
callers. By the way, it may fail when init() is inlined.

To fix this, CallersFrames() is used to translate PCs into symbolic
information accounting for inlined functions.

Test: go test ./...
Change-Id: I18b3c3ffdaf71f775e3522c87607aec5df7b09c5

package_ctx.go

@@ -158,8 +158,9 @@ func callerName(skip int) (pkgPath, funcName string, ok bool) {
 	if n != 1 {
 		return "", "", false
 	}
-
-	f := runtime.FuncForPC(pc[0]).Name()
+	frames := runtime.CallersFrames(pc[:])
+	frame, _ := frames.Next()
+	f := frame.Function
 	s := pkgPathRe.FindStringSubmatch(f)
 	if len(s) < 3 {
 		panic(fmt.Errorf("failed to extract package path and function name from %q", f))