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/legacy.go

@@ -6,6 +6,7 @@ import (
 	"net"
 
 	"github.com/lucas-clemente/quic-go"
+	"github.com/ooni/probe-cli/v3/internal/model"
 )
 
 // These vars export internal names to legacy ooni/probe-cli code.
@@ -51,7 +52,7 @@ type ResolverLegacy interface {
 
 // NewResolverLegacyAdapter adapts a ResolverLegacy to
 // become compatible with the Resolver definition.
-func NewResolverLegacyAdapter(reso ResolverLegacy) Resolver {
+func NewResolverLegacyAdapter(reso ResolverLegacy) model.Resolver {
 	return &ResolverLegacyAdapter{reso}
 }
 
@@ -60,7 +61,7 @@ type ResolverLegacyAdapter struct {
 	ResolverLegacy
 }
 
-var _ Resolver = &ResolverLegacyAdapter{}
+var _ model.Resolver = &ResolverLegacyAdapter{}
 
 type resolverLegacyNetworker interface {
 	Network() string
@@ -99,7 +100,7 @@ func (r *ResolverLegacyAdapter) CloseIdleConnections() {
 
 // LookupHTTPS always returns ErrDNSNoTransport.
 func (r *ResolverLegacyAdapter) LookupHTTPS(
-	ctx context.Context, domain string) (*HTTPSSvc, error) {
+	ctx context.Context, domain string) (*model.HTTPSSvc, error) {
 	return nil, ErrNoDNSTransport
 }
 
@@ -118,7 +119,7 @@ type DialerLegacy interface {
 // become compatible with the Dialer definition.
 //
 // Deprecated: do not use this function in new code.
-func NewDialerLegacyAdapter(d DialerLegacy) Dialer {
+func NewDialerLegacyAdapter(d DialerLegacy) model.Dialer {
 	return &DialerLegacyAdapter{d}
 }
 
@@ -130,7 +131,7 @@ type DialerLegacyAdapter struct {
 	DialerLegacy
 }
 
-var _ Dialer = &DialerLegacyAdapter{}
+var _ model.Dialer = &DialerLegacyAdapter{}
 
 type dialerLegacyIdleConnectionsCloser interface {
 	CloseIdleConnections()
@@ -159,7 +160,7 @@ type QUICContextDialer interface {
 
 // NewQUICDialerFromContextDialerAdapter creates a new
 // QUICDialer from a QUICContextDialer.
-func NewQUICDialerFromContextDialerAdapter(d QUICContextDialer) QUICDialer {
+func NewQUICDialerFromContextDialerAdapter(d QUICContextDialer) model.QUICDialer {
 	return &QUICContextDialerAdapter{d}
 }