refactor: move tracex outside of engine/netx (#782)

* refactor: move tracex outside of engine/netx

Consistently with https://github.com/ooni/probe/issues/2121 and
https://github.com/ooni/probe/issues/2115, we can now move tracex
outside of engine/netx. The main reason why this makes sense now
is that the package is now changed significantly from the one
that we imported from ooni/probe-engine.

We have improved its implementation, which had not been touched
significantly for quite some time, and converted it to unit
testing. I will document tomorrow some extra work I'd like to
do with this package but likely could not do $soon.

* go fmt

* regen tutorials

internal/tutorial/netxlite/chapter06/README.md

@@ -53,7 +53,7 @@ creating a system resolver, except that we also need to specify the
 UDP endpoint address at which the server is listening.
 
 ```Go
-	reso := netxlite.NewResolverUDP(log.Log, dialer, *serverAddr)
+	reso := netxlite.NewParallelResolverUDP(log.Log, dialer, *serverAddr)
 ```
 
 The API we invoke is the same as in the previous chapter, though,