refactor(netxlite): use *Netx for creating HTTP transports

This diff is like 50279a7 but uses *Netx to create HTTP transports.

The general idea of this patchset is to ensure we're not using duplicate code for constructing netxlite types, which is good to do now, because we're about to introduce new netxlite types for the network with which we communicate with the OONI backend.

Reference issue: ooni/probe#2531

Author: bassosimone

internal/netxlite/http.go

@@ -342,12 +342,19 @@ func (c *httpTLSConnWithReadTimeout) Read(b []byte) (int, error) {
 //
 // This factory and NewHTTPTransport are the recommended
 // ways of creating a new HTTPTransport.
-func NewHTTPTransportStdlib(logger model.DebugLogger) model.HTTPTransport {
-	dialer := NewDialerWithResolver(logger, NewStdlibResolver(logger))
-	tlsDialer := NewTLSDialer(dialer, NewTLSHandshakerStdlib(logger))
+func (netx *Netx) NewHTTPTransportStdlib(logger model.DebugLogger) model.HTTPTransport {
+	dialer := netx.NewDialerWithResolver(logger, netx.NewStdlibResolver(logger))
+	tlsDialer := NewTLSDialer(dialer, netx.NewTLSHandshakerStdlib(logger))
 	return NewHTTPTransport(logger, dialer, tlsDialer)
 }
 
+// NewHTTPTransportStdlib is equivalent to creating an empty [*Netx]
+// and calling its NewHTTPTransportStdlib method.
+func NewHTTPTransportStdlib(logger model.DebugLogger) model.HTTPTransport {
+	netx := &Netx{Underlying: nil}
+	return netx.NewHTTPTransportStdlib(logger)
+}
+
 // NewHTTPClientStdlib creates a new HTTPClient that uses the
 // standard library for TLS and DNS resolutions.
 func NewHTTPClientStdlib(logger model.DebugLogger) model.HTTPClient {

internal/netxlite/netx.go

@@ -22,14 +22,6 @@ func (netx *Netx) maybeCustomUnderlyingNetwork() *MaybeCustomUnderlyingNetwork {
 	return &MaybeCustomUnderlyingNetwork{netx.Underlying}
 }
 
-// NewHTTPTransportStdlib is like [netxlite.NewHTTPTransportStdlib] but the constructed [model.HTTPTransport]
-// uses the [model.UnderlyingNetwork] configured inside the [Netx] structure.
-func (n *Netx) NewHTTPTransportStdlib(logger model.DebugLogger) model.HTTPTransport {
-	dialer := n.NewDialerWithResolver(logger, n.NewStdlibResolver(logger))
-	tlsDialer := NewTLSDialer(dialer, n.NewTLSHandshakerStdlib(logger))
-	return NewHTTPTransport(logger, dialer, tlsDialer)
-}
-
 // NewHTTP3TransportStdlib is like [netxlite.NewHTTP3TransportStdlib] but the constructed [model.HTTPTransport]
 // uses the [model.UnderlyingNetwork] configured inside the [Netx] structure.
 func (n *Netx) NewHTTP3TransportStdlib(logger model.DebugLogger) model.HTTPTransport {