As of Go 1.9, ./... excludes the vendor directory by default and so we
no longer need to jump through hoops to exclude vendored packages from
testing, vetting, etc.

As a simplification this also re-introduces builtin/bins to the set of
packages we run tests on. With the providers now split into their own
repositories there's far fewer of these and so including them doesn't
really hurt anything, and makes our invocations here simpler.