refactor(netx): merge archival, trace, and the savers (#772)

This diff creates a new package under netx called tracex that
contains everything we need to perform measurements using events
tracing and postprocessing (which is the technique with which
we implement most network experiments).

The general idea here is to (1) create a unique package out of
all of these packages; (2) clean up the code a bit (improve tests,
docs, apply more recent code patterns); (3) move the resulting
code as a toplevel package inside of internal.

Once this is done, netx can be further refactored to avoid
subpackages and we can search for more code to salvage/refactor.

See https://github.com/ooni/probe/issues/2121

internal/cmd/oohelperd/internal/webconnectivity/dns.go

@@ -5,13 +5,13 @@ import (
 	"sync"
 
 	"github.com/ooni/probe-cli/v3/internal/engine/experiment/webconnectivity"
-	"github.com/ooni/probe-cli/v3/internal/engine/netx/archival"
+	"github.com/ooni/probe-cli/v3/internal/engine/netx/tracex"
 	"github.com/ooni/probe-cli/v3/internal/model"
 	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
 
 // newfailure is a convenience shortcut to save typing
-var newfailure = archival.NewFailure
+var newfailure = tracex.NewFailure
 
 // CtrlDNSResult is the result of the DNS check performed by
 // the Web Connectivity test helper.

internal/cmd/oohelperd/internal/webconnectivity/http.go

@@ -8,7 +8,7 @@ import (
 	"sync"
 
 	"github.com/ooni/probe-cli/v3/internal/engine/experiment/webconnectivity"
-	"github.com/ooni/probe-cli/v3/internal/engine/netx/archival"
+	"github.com/ooni/probe-cli/v3/internal/engine/netx/tracex"
 	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
 
@@ -81,7 +81,7 @@ func HTTPDo(ctx context.Context, config *HTTPConfig) {
 // See https://github.com/ooni/backend/blob/6ec4fda5b18/oonib/testhelpers/http_helpers.py#L361
 func httpMapFailure(err error) *string {
 	failure := newfailure(err)
-	failedOperation := archival.NewFailedOperation(err)
+	failedOperation := tracex.NewFailedOperation(err)
 	switch failure {
 	case nil:
 		return nil