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/kvstore/doc.go

@@ -0,0 +1,2 @@
+// Package kvstore implements model.KeyValueStore.
+package kvstore

internal/kvstore/fs.go

@@ -7,6 +7,7 @@ import (
 	"os"
 	"path/filepath"
 
+	"github.com/ooni/probe-cli/v3/internal/model"
 	"github.com/rogpeppe/go-internal/lockedfile"
 )
 
@@ -15,6 +16,8 @@ type FS struct {
 	basedir string
 }
 
+var _ model.KeyValueStore = &FS{}
+
 // NewFS creates a new kvstore.FileSystem.
 func NewFS(basedir string) (kvs *FS, err error) {
 	return newFileSystem(basedir, os.MkdirAll)

internal/kvstore/memory.go

@@ -1,9 +1,10 @@
-// Package kvstore contains key-value stores.
 package kvstore
 
 import (
 	"errors"
 	"sync"
+
+	"github.com/ooni/probe-cli/v3/internal/model"
 )
 
 // ErrNoSuchKey indicates that there's no value for the given key.
@@ -18,6 +19,8 @@ type Memory struct {
 	mu sync.Mutex
 }
 
+var _ model.KeyValueStore = &Memory{}
+
 // Get returns the specified key's value. In case of error, the
 // error type is such that errors.Is(err, ErrNoSuchKey).
 func (kvs *Memory) Get(key string) ([]byte, error) {