The ztypes_* file names indicate that these are auto-generated while in
fact they aren't. Rename them to types_* to avoid this confusion.

Noticed by Brad Fitzpatrick in https://golang.org/cl/52670.

Change-Id: I75c8d0f4116dfb3a24ae9444509daf31524c35d6
Reviewed-on: https://go-review.googlesource.com/52950Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>