cleanup: remove unnecessary legacy interfaces (#656)

This diff addresses another point of https://github.com/ooni/probe/issues/1956:

> - [ ] observe that we're still using a bunch of private interfaces for common interfaces such as the `Dialer`, so we can get rid of these private interfaces and always use the ones in `model`, which allows us to remove a bunch of legacy wrappers

Additional cleanups may still be possible. The more I cleanup, the more I see
there's extra legacy code we can dispose of (which seems good?).

internal/engine/experiment/dnscheck/dnscheck.go

@@ -234,7 +234,7 @@ func (m *Measurer) Run(
 	return nil
 }
 
-func (m *Measurer) lookupHost(ctx context.Context, hostname string, r netx.Resolver) ([]string, error) {
+func (m *Measurer) lookupHost(ctx context.Context, hostname string, r model.Resolver) ([]string, error) {
 	ctx, cancel := context.WithTimeout(ctx, 20*time.Second)
 	defer cancel()
 	return r.LookupHost(ctx, hostname)