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/oohelper/internal/fake_test.go

@@ -2,13 +2,14 @@ package internal
 
 import (
 	"context"
+	"errors"
 	"io"
 	"net"
 	"net/http"
 	"time"
 
 	"github.com/ooni/probe-cli/v3/internal/atomicx"
-	"github.com/ooni/probe-cli/v3/internal/engine/netx"
+	"github.com/ooni/probe-cli/v3/internal/model"
 	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
 
@@ -49,7 +50,13 @@ func (c FakeResolver) Address() string {
 	return ""
 }
 
-var _ netx.Resolver = FakeResolver{}
+func (c FakeResolver) CloseIdleConnections() {}
+
+func (c FakeResolver) LookupHTTPS(ctx context.Context, domain string) (*model.HTTPSSvc, error) {
+	return nil, errors.New("not implemented")
+}
+
+var _ model.Resolver = FakeResolver{}
 
 type FakeTransport struct {
 	Err  error
@@ -75,7 +82,7 @@ func (txp FakeTransport) RoundTrip(req *http.Request) (*http.Response, error) {
 
 func (txp FakeTransport) CloseIdleConnections() {}
 
-var _ netx.HTTPRoundTripper = FakeTransport{}
+var _ model.HTTPTransport = FakeTransport{}
 
 type FakeBody struct {
 	Data []byte

internal/cmd/oohelper/oohelper.go

@@ -14,6 +14,7 @@ import (
 	"github.com/ooni/probe-cli/v3/internal/engine/experiment/webstepsx"
 	"github.com/ooni/probe-cli/v3/internal/engine/netx"
 	"github.com/ooni/probe-cli/v3/internal/measurex"
+	"github.com/ooni/probe-cli/v3/internal/model"
 	"github.com/ooni/probe-cli/v3/internal/runtimex"
 )
 
@@ -21,7 +22,7 @@ var (
 	ctx, cancel = context.WithCancel(context.Background())
 	debug       = flag.Bool("debug", false, "Toggle debug mode")
 	httpClient  *http.Client
-	resolver    netx.Resolver
+	resolver    model.Resolver
 	server      = flag.String("server", "", "URL of the test helper")
 	target      = flag.String("target", "", "Target URL for the test helper")
 	fwebsteps   = flag.Bool("websteps", false, "Use the websteps TH")