cmd/vet: use type information in isLocalType

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>