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/httptransport/httptransport.go

@@ -2,46 +2,15 @@
 package httptransport
 
 import (
-	"context"
 	"crypto/tls"
-	"net"
-	"net/http"
 
-	"github.com/lucas-clemente/quic-go"
+	"github.com/ooni/probe-cli/v3/internal/model"
 )
 
 // Config contains the configuration required for constructing an HTTP transport
 type Config struct {
-	Dialer     Dialer
-	QUICDialer QUICDialer
-	TLSDialer  TLSDialer
+	Dialer     model.Dialer
+	QUICDialer model.QUICDialer
+	TLSDialer  model.TLSDialer
 	TLSConfig  *tls.Config
 }
-
-// Dialer is the definition of dialer assumed by this package.
-type Dialer interface {
-	DialContext(ctx context.Context, network, address string) (net.Conn, error)
-}
-
-// TLSDialer is the definition of a TLS dialer assumed by this package.
-type TLSDialer interface {
-	DialTLSContext(ctx context.Context, network, address string) (net.Conn, error)
-}
-
-// QUICDialer is the definition of dialer for QUIC assumed by this package.
-type QUICDialer interface {
-	DialContext(ctx context.Context, network, addr string, tlsCfg *tls.Config, cfg *quic.Config) (quic.EarlySession, error)
-}
-
-// RoundTripper is the definition of http.RoundTripper used by this package.
-type RoundTripper interface {
-	RoundTrip(req *http.Request) (*http.Response, error)
-	CloseIdleConnections()
-}
-
-// Resolver is the interface we expect from a resolver
-type Resolver interface {
-	LookupHost(ctx context.Context, hostname string) (addrs []string, err error)
-	Network() string
-	Address() string
-}