Now that vet always has type information, there's no reason to use
string handling on type names to gather information about them, such as
whether or not they are a local type.

The semantics remain the same - the only difference should be that the
implementation is less fragile and simpler.

Change-Id: I71386b4196922e4c9f2653d90abc382efbf01b3c
Reviewed-on: https://go-review.googlesource.com/95915
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>