gosrc: regenerate with Go 1.13

This change regenerates the list of standard packages to contain
packages that are in Go 1.13, which has been recently released.¹

It was done by running:

	go generate ./gosrc

¹ https://groups.google.com/d/msg/golang-announce/wQqVlKJiMBA/HqXJXZSuBwAJ

Updates golang/gddo#644

Change-Id: I701bb6ee2929996c1572d98dce5fd053d35ddb50
Reviewed-on: https://go-review.googlesource.com/c/gddo/+/193257
Reviewed-by: Tuo Shan <shantuo@google.com>