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/netxlite/dialer.go

@@ -6,19 +6,12 @@ import (
 	"net"
 	"sync"
 	"time"
+
+	"github.com/ooni/probe-cli/v3/internal/model"
 )
 
-// Dialer establishes network connections.
-type Dialer interface {
-	// DialContext behaves like net.Dialer.DialContext.
-	DialContext(ctx context.Context, network, address string) (net.Conn, error)
-
-	// CloseIdleConnections closes idle connections, if any.
-	CloseIdleConnections()
-}
-
 // NewDialerWithResolver calls WrapDialer for the stdlib dialer.
-func NewDialerWithResolver(logger Logger, resolver Resolver) Dialer {
+func NewDialerWithResolver(logger model.DebugLogger, resolver model.Resolver) model.Dialer {
 	return WrapDialer(logger, resolver, &dialerSystem{})
 }
 
@@ -51,26 +44,26 @@ func NewDialerWithResolver(logger Logger, resolver Resolver) Dialer {
 //
 // In general, do not use WrapDialer directly but try to use
 // more high-level factories, e.g., NewDialerWithResolver.
-func WrapDialer(logger Logger, resolver Resolver, dialer Dialer) Dialer {
+func WrapDialer(logger model.DebugLogger, resolver model.Resolver, dialer model.Dialer) model.Dialer {
 	return &dialerLogger{
 		Dialer: &dialerResolver{
 			Dialer: &dialerLogger{
 				Dialer: &dialerErrWrapper{
 					Dialer: dialer,
 				},
-				Logger:          logger,
+				DebugLogger:     logger,
 				operationSuffix: "_address",
 			},
 			Resolver: resolver,
 		},
-		Logger: logger,
+		DebugLogger: logger,
 	}
 }
 
 // NewDialerWithoutResolver calls NewDialerWithResolver with a "null" resolver.
 //
 // The returned dialer fails with ErrNoResolver if passed a domain name.
-func NewDialerWithoutResolver(logger Logger) Dialer {
+func NewDialerWithoutResolver(logger model.DebugLogger) model.Dialer {
 	return NewDialerWithResolver(logger, &nullResolver{})
 }
 
@@ -81,16 +74,16 @@ type dialerSystem struct {
 	timeout time.Duration
 }
 
-var _ Dialer = &dialerSystem{}
+var _ model.Dialer = &dialerSystem{}
 
 const dialerDefaultTimeout = 15 * time.Second
 
-func (d *dialerSystem) newUnderlyingDialer() TProxyDialer {
+func (d *dialerSystem) newUnderlyingDialer() model.SimpleDialer {
 	t := d.timeout
 	if t <= 0 {
 		t = dialerDefaultTimeout
 	}
-	return TProxy.NewTProxyDialer(t)
+	return TProxy.NewSimpleDialer(t)
 }
 
 func (d *dialerSystem) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
@@ -103,11 +96,11 @@ func (d *dialerSystem) CloseIdleConnections() {
 
 // dialerResolver combines dialing with domain name resolution.
 type dialerResolver struct {
-	Dialer
-	Resolver
+	model.Dialer
+	model.Resolver
 }
 
-var _ Dialer = &dialerResolver{}
+var _ model.Dialer = &dialerResolver{}
 
 func (d *dialerResolver) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
 	// QUIRK: this routine and the related routines in quirks.go cannot
@@ -151,10 +144,10 @@ func (d *dialerResolver) CloseIdleConnections() {
 // dialerLogger is a Dialer with logging.
 type dialerLogger struct {
 	// Dialer is the underlying dialer.
-	Dialer
+	model.Dialer
 
 	// Logger is the underlying logger.
-	Logger
+	model.DebugLogger
 
 	// operationSuffix is appended to the operation name.
 	//
@@ -165,19 +158,19 @@ type dialerLogger struct {
 	operationSuffix string
 }
 
-var _ Dialer = &dialerLogger{}
+var _ model.Dialer = &dialerLogger{}
 
 func (d *dialerLogger) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
-	d.Logger.Debugf("dial%s %s/%s...", d.operationSuffix, address, network)
+	d.DebugLogger.Debugf("dial%s %s/%s...", d.operationSuffix, address, network)
 	start := time.Now()
 	conn, err := d.Dialer.DialContext(ctx, network, address)
 	elapsed := time.Since(start)
 	if err != nil {
-		d.Logger.Debugf("dial%s %s/%s... %s in %s", d.operationSuffix,
+		d.DebugLogger.Debugf("dial%s %s/%s... %s in %s", d.operationSuffix,
 			address, network, err, elapsed)
 		return nil, err
 	}
-	d.Logger.Debugf("dial%s %s/%s... ok in %s", d.operationSuffix,
+	d.DebugLogger.Debugf("dial%s %s/%s... ok in %s", d.operationSuffix,
 		address, network, elapsed)
 	return conn, nil
 }
@@ -195,7 +188,7 @@ var ErrNoConnReuse = errors.New("cannot reuse connection")
 // dial will succed and return conn regardless of the network
 // and address arguments passed to DialContext. Any subsequent
 // dial returns ErrNoConnReuse.
-func NewSingleUseDialer(conn net.Conn) Dialer {
+func NewSingleUseDialer(conn net.Conn) model.Dialer {
 	return &dialerSingleUse{conn: conn}
 }
 
@@ -205,7 +198,7 @@ type dialerSingleUse struct {
 	conn net.Conn
 }
 
-var _ Dialer = &dialerSingleUse{}
+var _ model.Dialer = &dialerSingleUse{}
 
 func (s *dialerSingleUse) DialContext(ctx context.Context, network string, addr string) (net.Conn, error) {
 	defer s.Unlock()
@@ -225,10 +218,10 @@ func (s *dialerSingleUse) CloseIdleConnections() {
 // dialerErrWrapper is a dialer that performs error wrapping. The connection
 // returned by the DialContext function will also perform error wrapping.
 type dialerErrWrapper struct {
-	Dialer
+	model.Dialer
 }
 
-var _ Dialer = &dialerErrWrapper{}
+var _ model.Dialer = &dialerErrWrapper{}
 
 func (d *dialerErrWrapper) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
 	conn, err := d.Dialer.DialContext(ctx, network, address)
@@ -274,13 +267,13 @@ func (c *dialerErrWrapperConn) Close() error {
 var ErrNoDialer = errors.New("no configured dialer")
 
 // NewNullDialer returns a dialer that always fails with ErrNoDialer.
-func NewNullDialer() Dialer {
+func NewNullDialer() model.Dialer {
 	return &nullDialer{}
 }
 
 type nullDialer struct{}
 
-var _ Dialer = &nullDialer{}
+var _ model.Dialer = &nullDialer{}
 
 func (*nullDialer) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
 	return nil, ErrNoDialer