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.go

@@ -0,0 +1,60 @@
+package mocks
+
+import (
+	"context"
+	"crypto/tls"
+	"net"
+)
+
+// TLSHandshaker is a mockable TLS handshaker.
+type TLSHandshaker struct {
+	MockHandshake func(ctx context.Context, conn net.Conn, config *tls.Config) (
+		net.Conn, tls.ConnectionState, error)
+}
+
+// Handshake calls MockHandshake.
+func (th *TLSHandshaker) Handshake(ctx context.Context, conn net.Conn, config *tls.Config) (
+	net.Conn, tls.ConnectionState, error) {
+	return th.MockHandshake(ctx, conn, config)
+}
+
+// TLSConn allows to mock netxlite.TLSConn.
+type TLSConn struct {
+	// Conn is the embedded mockable Conn.
+	Conn
+
+	// MockConnectionState allows to mock the ConnectionState method.
+	MockConnectionState func() tls.ConnectionState
+
+	// MockHandshakeContext allows to mock the HandshakeContext method.
+	MockHandshakeContext func(ctx context.Context) error
+}
+
+// ConnectionState calls MockConnectionState.
+func (c *TLSConn) ConnectionState() tls.ConnectionState {
+	return c.MockConnectionState()
+}
+
+// HandshakeContext calls MockHandshakeContext.
+func (c *TLSConn) HandshakeContext(ctx context.Context) error {
+	return c.MockHandshakeContext(ctx)
+}
+
+// TLSDialer allows to mock netxlite.TLSDialer.
+type TLSDialer struct {
+	// MockCloseIdleConnections allows to mock the CloseIdleConnections method.
+	MockCloseIdleConnections func()
+
+	// MockDialTLSContext allows to mock the DialTLSContext method.
+	MockDialTLSContext func(ctx context.Context, network, address string) (net.Conn, error)
+}
+
+// CloseIdleConnections calls MockCloseIdleConnections.
+func (d *TLSDialer) CloseIdleConnections() {
+	d.MockCloseIdleConnections()
+}
+
+// DialTLSContext calls MockDialTLSContext.
+func (d *TLSDialer) DialTLSContext(ctx context.Context, network, address string) (net.Conn, error) {
+	return d.MockDialTLSContext(ctx, network, address)
+}