[Go style says](https://blog.golang.org/package-names):
> Avoid stutter. Since client code uses the package name as a prefix
> when referring to the package contents, the names for those contents
> need not repeat the package name. The HTTP server provided by the
> http package is called Server, not HTTPServer. Client code refers to
> this type as http.Server, so there is no ambiguity.
Rename DNSClient, DNSClientImpl, NewDNSClientImpl,
NewTestDNSClientImpl, DNSError, and MockDNSClient to follow those
guidelines.
Unexport DNSClientImpl and MockTimeoutError (was only used internally).
Make New and NewTest return the Client interface rather than a concrete
`impl` type.
2a8f0fe6ac