cmd/vet: copy changes from golang.org/x/tools to cmd/vet

This means bringing over the examples flag and sorting doc.go.

Subsequent changes will generalize the examples flag to a general
test naming flag, but let's start with the original code.

No more changes to golang.org/x/tools please. This should not have
happened (and letting it happen was partly my fault).

Change-Id: Ia879ea1d15d82372df14853f919263125dfb7b96
Reviewed-on: https://go-review.googlesource.com/14821


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>