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/experiment/ndt7/dial.go

@@ -30,7 +30,7 @@ func newDialManager(ndt7URL string, logger model.Logger, userAgent string) dialM
 }
 
 func (mgr dialManager) dialWithTestName(ctx context.Context, testName string) (*websocket.Conn, error) {
-	reso := netxlite.NewResolverStdlib(mgr.logger)
+	reso := netxlite.NewStdlibResolver(mgr.logger)
 	dlr := netxlite.NewDialerWithResolver(mgr.logger, reso)
 	dlr = bytecounter.WrapWithContextAwareDialer(dlr)
 	// Implements shaping if the user builds using `-tags shaping`