refactor(netxlite): expose useful HTTPTransport/DNSTransport factories (#813)

These factories will soon be useful to finish with
https://github.com/ooni/probe/issues/2135.

internal/cmd/oohelper/oohelper.go

@@ -29,8 +29,7 @@ func init() {
 	// puzzling https://github.com/ooni/probe/issues/1409 issue.
 	const resolverURL = "https://8.8.8.8/dns-query"
 	resolver = netxlite.NewParallelDNSOverHTTPSResolver(log.Log, resolverURL)
-	txp := netxlite.NewHTTPTransportWithResolver(log.Log, resolver)
-	httpClient = netxlite.NewHTTPClient(txp)
+	httpClient = netxlite.NewHTTPClientWithResolver(log.Log, resolver)
 }
 
 func main() {

internal/cmd/oohelperd/oohelperd.go

@@ -32,8 +32,7 @@ func init() {
 	// default resolver configured by the box. Also, use an encrypted transport thus
 	// we're less vulnerable to any policy implemented by the box's provider.
 	resolver = netxlite.NewParallelDNSOverHTTPSResolver(log.Log, "https://8.8.8.8/dns-query")
-	txp := netxlite.NewHTTPTransportWithResolver(log.Log, resolver)
-	httpClient = netxlite.NewHTTPClient(txp)
+	httpClient = netxlite.NewHTTPClientWithResolver(log.Log, resolver)
 }
 
 func shutdown(srv *http.Server) {