cmd/godex: handle printing of type aliases

Also: remove go1.5 build tags (x/tools requires Go 1.6 or higher).

For golang/go#18130.

Change-Id: I3d9deee9e87d8794b2884281c0bb53caa5ed6221
Reviewed-on: https://go-review.googlesource.com/35106
Reviewed-by: Alan Donovan <adonovan@google.com>
8 files changed