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/netx/resolver/cache.go

@@ -3,12 +3,14 @@ package resolver
 import (
 	"context"
 	"sync"
+
+	"github.com/ooni/probe-cli/v3/internal/model"
 )
 
 // CacheResolver is a resolver that caches successful replies.
 type CacheResolver struct {
 	ReadOnly bool
-	Resolver
+	model.Resolver
 	mu    sync.Mutex
 	cache map[string][]string
 }