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/measurex/logger.go

@@ -11,24 +11,12 @@ import (
 	"sync"
 	"time"
 
-	"github.com/ooni/probe-cli/v3/internal/netxlite"
+	"github.com/ooni/probe-cli/v3/internal/model"
 )
 
-// Logger is the logger type we use. This type is compatible
-// with the logger type of github.com/apex/log.
-type Logger interface {
-	netxlite.Logger
-
-	Info(msg string)
-	Infof(format string, v ...interface{})
-
-	Warn(msg string)
-	Warnf(format string, v ...interface{})
-}
-
 // NewOperationLogger creates a new logger that logs
 // about an in-progress operation.
-func NewOperationLogger(logger Logger, format string, v ...interface{}) *OperationLogger {
+func NewOperationLogger(logger model.Logger, format string, v ...interface{}) *OperationLogger {
 	ol := &OperationLogger{
 		sighup:  make(chan interface{}),
 		logger:  logger,
@@ -43,7 +31,7 @@ func NewOperationLogger(logger Logger, format string, v ...interface{}) *Operati
 
 // OperationLogger logs about an in-progress operation
 type OperationLogger struct {
-	logger  Logger
+	logger  model.Logger
 	message string
 	once    *sync.Once
 	sighup  chan interface{}