cleanup: netx does not use netxlite legacy names (#801)

This diff refactors netx and netxlite to ensure we're not using
netxlite legacy names inside of netx.

To this end, we're cheating a bit. We're exposing a new factory to
get an unwrapped stdlib resolver rather than defining a legacy name
to export the private name of the same factory.

This is actually a fine place to stop, for now, the next and
netxlite refactoring at https://github.com/ooni/probe/issues/2121.

internal/engine/netx/dnstransport.go

@@ -65,7 +65,7 @@ func NewDNSClientWithOverrides(config Config, URL, hostOverride, SNIOverride,
 	}
 	switch resolverURL.Scheme {
 	case "system":
-		return netxlite.NewResolverSystem(), nil
+		return netxlite.NewUnwrappedStdlibResolver(), nil
 	case "https":
 		config.TLSConfig.NextProtos = []string{"h2", "http/1.1"}
 		httpClient := &http.Client{Transport: NewHTTPTransport(config)}

internal/engine/netx/resolver.go

@@ -12,7 +12,7 @@ import (
 // NewResolver creates a new resolver from the specified config.
 func NewResolver(config Config) model.Resolver {
 	if config.BaseResolver == nil {
-		config.BaseResolver = netxlite.NewResolverSystem()
+		config.BaseResolver = netxlite.NewUnwrappedStdlibResolver()
 	}
 	r := netxlite.WrapResolver(
 		model.ValidLoggerOrDefault(config.Logger),