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/model/mocks/tls_test.go

@@ -0,0 +1,95 @@
+package mocks
+
+import (
+	"context"
+	"crypto/tls"
+	"errors"
+	"net"
+	"reflect"
+	"testing"
+)
+
+func TestTLSHandshaker(t *testing.T) {
+	t.Run("Handshake", func(t *testing.T) {
+		expected := errors.New("mocked error")
+		conn := &Conn{}
+		ctx := context.Background()
+		config := &tls.Config{}
+		th := &TLSHandshaker{
+			MockHandshake: func(ctx context.Context, conn net.Conn,
+				config *tls.Config) (net.Conn, tls.ConnectionState, error) {
+				return nil, tls.ConnectionState{}, expected
+			},
+		}
+		tlsConn, connState, err := th.Handshake(ctx, conn, config)
+		if !errors.Is(err, expected) {
+			t.Fatal("not the error we expected", err)
+		}
+		if !reflect.ValueOf(connState).IsZero() {
+			t.Fatal("expected zero ConnectionState here")
+		}
+		if tlsConn != nil {
+			t.Fatal("expected nil conn here")
+		}
+	})
+}
+
+func TestTLSConn(t *testing.T) {
+	t.Run("ConnectionState", func(t *testing.T) {
+		state := tls.ConnectionState{Version: tls.VersionTLS12}
+		c := &TLSConn{
+			MockConnectionState: func() tls.ConnectionState {
+				return state
+			},
+		}
+		out := c.ConnectionState()
+		if !reflect.DeepEqual(out, state) {
+			t.Fatal("not the result we expected")
+		}
+	})
+
+	t.Run("HandshakeContext", func(t *testing.T) {
+		expected := errors.New("mocked error")
+		c := &TLSConn{
+			MockHandshakeContext: func(ctx context.Context) error {
+				return expected
+			},
+		}
+		err := c.HandshakeContext(context.Background())
+		if !errors.Is(err, expected) {
+			t.Fatal("not the error we expected", err)
+		}
+	})
+}
+
+func TestTLSDialer(t *testing.T) {
+	t.Run("CloseIdleConnections", func(t *testing.T) {
+		var called bool
+		td := &TLSDialer{
+			MockCloseIdleConnections: func() {
+				called = true
+			},
+		}
+		td.CloseIdleConnections()
+		if !called {
+			t.Fatal("not called")
+		}
+	})
+
+	t.Run("DialTLSContext", func(t *testing.T) {
+		expected := errors.New("mocked error")
+		td := &TLSDialer{
+			MockDialTLSContext: func(ctx context.Context, network, address string) (net.Conn, error) {
+				return nil, expected
+			},
+		}
+		ctx := context.Background()
+		conn, err := td.DialTLSContext(ctx, "", "")
+		if !errors.Is(err, expected) {
+			t.Fatal("not the error we expected", err)
+		}
+		if conn != nil {
+			t.Fatal("expected nil conn here")
+		}
+	})
+}