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/mlablocate/mlablocate.go

@@ -12,15 +12,10 @@ import (
 	"net/http"
 	"net/url"
 
+	"github.com/ooni/probe-cli/v3/internal/model"
 	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
 
-// Logger is the logger expected by this package.
-type Logger interface {
-	// Debugf formats and emits a debug message.
-	Debugf(format string, v ...interface{})
-}
-
 // HTTPClient is anything that looks like an http.Client.
 type HTTPClient interface {
 	// Do behaves like http.Client.Do.
@@ -38,7 +33,7 @@ type Client struct {
 	Hostname string
 
 	// Logger is the MANDATORY logger to use.
-	Logger Logger
+	Logger model.DebugLogger
 
 	// Scheme is the MANDATORY scheme to use (http or https).
 	Scheme string
@@ -48,7 +43,7 @@ type Client struct {
 }
 
 // NewClient creates a new locate.measurementlab.net client.
-func NewClient(httpClient HTTPClient, logger Logger, userAgent string) *Client {
+func NewClient(httpClient HTTPClient, logger model.DebugLogger, userAgent string) *Client {
 	return &Client{
 		HTTPClient: httpClient,
 		Hostname:   "locate.measurementlab.net",