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/tproxy.go

@@ -5,46 +5,22 @@ import (
 	"net"
 	"time"
 
-	"github.com/ooni/probe-cli/v3/internal/netxlite/quicx"
+	"github.com/ooni/probe-cli/v3/internal/model"
 )
 
-// TProxable is the fundamental type used by the netxlite package to perform
-// low-level network operations for which, by default, we use the stdlib.
-//
-// The t stands for transparent. By using this type as the fundamental type,
-// we can transparently intercept connections and implement censorship
-// policies. The implementation of this functionality is not part of netxlite:
-// here we only have the basic mechanism to make this possible.
-type TProxable interface {
-	// ListenUDP creates a new quicx.UDPLikeConn conn.
-	ListenUDP(network string, laddr *net.UDPAddr) (quicx.UDPLikeConn, error)
-
-	// LookupHost lookups a domain using the stdlib resolver.
-	LookupHost(ctx context.Context, domain string) ([]string, error)
-
-	// NewTProxyDialer returns a new TProxyDialer.
-	NewTProxyDialer(timeout time.Duration) TProxyDialer
-}
-
-// TProxyDialer is the dialer type returned by TProxable.NewDialer.
-type TProxyDialer interface {
-	// DialContext behaves like net.Dialer.DialContext.
-	DialContext(ctx context.Context, network, address string) (net.Conn, error)
-}
-
 // TProxy is the fundamental variable controlling how netxlite creates
-// net.Conn and quicx.UDPLikeConn, as well as how it uses the stdlib
+// net.Conn and model.UDPLikeConn, as well as how it uses the stdlib
 // resolver. By modifying this variable, you can effectively transparently
 // proxy netxlite (and hence OONI) activities to other services. This is
 // quite convenient when performing quality assurance tests.
-var TProxy TProxable = &TProxyStdlib{}
+var TProxy model.UnderlyingNetworkLibrary = &TProxyStdlib{}
 
-// TProxyStdlib is the default TProxable implementation that uses
+// TProxyStdlib is the default model.UnderlyingNetworkLibrary using
 // the stdlib in the most obvious way for every functionality.
 type TProxyStdlib struct{}
 
 // ListenUDP calls net.ListenUDP.
-func (*TProxyStdlib) ListenUDP(network string, laddr *net.UDPAddr) (quicx.UDPLikeConn, error) {
+func (*TProxyStdlib) ListenUDP(network string, laddr *net.UDPAddr) (model.UDPLikeConn, error) {
 	return net.ListenUDP(network, laddr)
 }
 
@@ -53,7 +29,7 @@ func (*TProxyStdlib) LookupHost(ctx context.Context, domain string) ([]string, e
 	return net.DefaultResolver.LookupHost(ctx, domain)
 }
 
-// NewTProxyDialer returns a &net.Dialer{Timeout: timeout} instance.
-func (*TProxyStdlib) NewTProxyDialer(timeout time.Duration) TProxyDialer {
+// NewSimpleDialer returns a &net.Dialer{Timeout: timeout} instance.
+func (*TProxyStdlib) NewSimpleDialer(timeout time.Duration) model.SimpleDialer {
 	return &net.Dialer{Timeout: timeout}
 }