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/tutorial/netxlite/chapter05/README.md

@@ -51,7 +51,7 @@ The returned resolver implements an interface that is very
 close to the API of the `net.Resolver` struct.
 
 ```Go
-	reso := netxlite.NewResolverStdlib(log.Log)
+	reso := netxlite.NewStdlibResolver(log.Log)
 ```
 
 We call `LookupHost` to map the hostname to IP addrs. The returned

internal/tutorial/netxlite/chapter05/main.go

@@ -52,7 +52,7 @@ func main() {
 	// close to the API of the `net.Resolver` struct.
 	//
 	// ```Go
-	reso := netxlite.NewResolverStdlib(log.Log)
+	reso := netxlite.NewStdlibResolver(log.Log)
 	// ```
 	//
 	// We call `LookupHost` to map the hostname to IP addrs. The returned