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/experiment/torsf/chapter04/README.md

@@ -38,7 +38,7 @@ The `tracex` package contains code used to format internal
 measurements representations to the OONI data format.
 
 ```Go
-	"github.com/ooni/probe-cli/v3/internal/engine/netx/tracex"
+	"github.com/ooni/probe-cli/v3/internal/tracex"
 
 ```
 

internal/tutorial/experiment/torsf/chapter04/torsf.go

@@ -41,7 +41,7 @@ import (
 	// measurements representations to the OONI data format.
 	//
 	// ```Go
-	"github.com/ooni/probe-cli/v3/internal/engine/netx/tracex"
+	"github.com/ooni/probe-cli/v3/internal/tracex"
 
 	// ```
 	//