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/netxlite/dialer_test.go

@@ -141,7 +141,7 @@ func TestDialerResolver(t *testing.T) {
 		t.Run("fails without a port", func(t *testing.T) {
 			d := &dialerResolver{
 				Dialer:   &DialerSystem{},
-				Resolver: newResolverSystem(),
+				Resolver: NewUnwrappedStdlibResolver(),
 			}
 			const missingPort = "ooni.nu"
 			conn, err := d.DialContext(context.Background(), "tcp", missingPort)