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/cmd/oohelperd/internal/webconnectivity/measure.go

@@ -8,7 +8,7 @@ import (
 	"sync"
 
 	"github.com/ooni/probe-cli/v3/internal/engine/experiment/webconnectivity"
-	"github.com/ooni/probe-cli/v3/internal/engine/netx"
+	"github.com/ooni/probe-cli/v3/internal/model"
 )
 
 type (
@@ -22,9 +22,9 @@ type (
 // MeasureConfig contains configuration for Measure.
 type MeasureConfig struct {
 	Client            *http.Client
-	Dialer            netx.Dialer
+	Dialer            model.Dialer
 	MaxAcceptableBody int64
-	Resolver          netx.Resolver
+	Resolver          model.Resolver
 }
 
 // Measure performs the measurement described by the request and