refactor: interfaces and data types into the model package (#642)

## Checklist

- [x] I have read the [contribution guidelines](https://github.com/ooni/probe-cli/blob/master/CONTRIBUTING.md)
- [x] reference issue for this pull request: https://github.com/ooni/probe/issues/1885
- [x] related ooni/spec pull request: N/A

Location of the issue tracker: https://github.com/ooni/probe

## Description

This PR contains a set of changes to move important interfaces and data types into the `./internal/model` package.

The criteria for including an interface or data type in here is roughly that the type should be important and used by several packages. We are especially interested to move more interfaces here to increase modularity.

An additional side effect is that, by reading this package, one should be able to understand more quickly how different parts of the codebase interact with each other.

This is what I want to move in `internal/model`:

- [x] most important interfaces from `internal/netxlite`
- [x] everything that was previously part of `internal/engine/model`
- [x] mocks from `internal/netxlite/mocks` should also be moved in here as a subpackage

internal/engine/experiment/tor/tor.go

@@ -17,7 +17,7 @@ import (
 	"github.com/ooni/probe-cli/v3/internal/engine/legacy/netxlogger"
 	"github.com/ooni/probe-cli/v3/internal/engine/legacy/oonidatamodel"
 	"github.com/ooni/probe-cli/v3/internal/engine/legacy/oonitemplates"
-	"github.com/ooni/probe-cli/v3/internal/engine/model"
+	"github.com/ooni/probe-cli/v3/internal/model"
 	"github.com/ooni/probe-cli/v3/internal/netxlite"
 	"github.com/ooni/probe-cli/v3/internal/runtimex"
 	"github.com/ooni/probe-cli/v3/internal/scrubber"
@@ -144,14 +144,14 @@ func (tk *TestKeys) fillToplevelKeys() {
 // Measurer performs the measurement.
 type Measurer struct {
 	config          Config
-	fetchTorTargets func(ctx context.Context, sess model.ExperimentSession, cc string) (map[string]model.TorTarget, error)
+	fetchTorTargets func(ctx context.Context, sess model.ExperimentSession, cc string) (map[string]model.OOAPITorTarget, error)
 }
 
 // NewMeasurer creates a new Measurer
 func NewMeasurer(config Config) *Measurer {
 	return &Measurer{
 		config: config,
-		fetchTorTargets: func(ctx context.Context, sess model.ExperimentSession, cc string) (map[string]model.TorTarget, error) {
+		fetchTorTargets: func(ctx context.Context, sess model.ExperimentSession, cc string) (map[string]model.OOAPITorTarget, error) {
 			return sess.FetchTorTargets(ctx, cc)
 		},
 	}
@@ -189,7 +189,7 @@ func (m *Measurer) Run(
 
 func (m *Measurer) gimmeTargets(
 	ctx context.Context, sess model.ExperimentSession,
-) (map[string]model.TorTarget, error) {
+) (map[string]model.OOAPITorTarget, error) {
 	ctx, cancel := context.WithTimeout(ctx, 15*time.Second)
 	defer cancel()
 	return m.fetchTorTargets(ctx, sess, sess.ProbeCC())
@@ -198,7 +198,7 @@ func (m *Measurer) gimmeTargets(
 // keytarget contains a key and the related target
 type keytarget struct {
 	key    string
-	target model.TorTarget
+	target model.OOAPITorTarget
 }
 
 // private returns whether a target is private. We consider private
@@ -222,7 +222,7 @@ func (m *Measurer) measureTargets(
 	sess model.ExperimentSession,
 	measurement *model.Measurement,
 	callbacks model.ExperimentCallbacks,
-	targets map[string]model.TorTarget,
+	targets map[string]model.OOAPITorTarget,
 ) {
 	// run measurements in parallel
 	var waitgroup sync.WaitGroup
@@ -327,7 +327,7 @@ func maybeScrubbingLogger(input model.Logger, kt keytarget) model.Logger {
 	if !kt.private() {
 		return input
 	}
-	return &scrubber.Logger{UnderlyingLogger: input}
+	return &scrubber.Logger{Logger: input}
 }
 
 func (rc *resultsCollector) defaultFlexibleConnect(