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/quic.go

@@ -13,26 +13,17 @@ import (
 	"time"
 
 	"github.com/lucas-clemente/quic-go"
+	"github.com/ooni/probe-cli/v3/internal/model"
 	"github.com/ooni/probe-cli/v3/internal/netxlite"
-	"github.com/ooni/probe-cli/v3/internal/netxlite/quicx"
 )
 
-// QUICConn is the kind of conn used by QUIC.
-type QUICConn = quicx.UDPLikeConn
-
-// QUICDialer creates QUICSesssions.
-type QUICDialer = netxlite.QUICDialer
-
-// QUICListener creates listening connections for QUIC.
-type QUICListener = netxlite.QUICListener
-
 type quicListenerDB struct {
-	netxlite.QUICListener
+	model.QUICListener
 	begin time.Time
 	db    WritableDB
 }
 
-func (ql *quicListenerDB) Listen(addr *net.UDPAddr) (QUICConn, error) {
+func (ql *quicListenerDB) Listen(addr *net.UDPAddr) (model.UDPLikeConn, error) {
 	pconn, err := ql.QUICListener.Listen(addr)
 	if err != nil {
 		return nil, err
@@ -45,7 +36,7 @@ func (ql *quicListenerDB) Listen(addr *net.UDPAddr) (QUICConn, error) {
 }
 
 type udpLikeConnDB struct {
-	quicx.UDPLikeConn
+	model.UDPLikeConn
 	begin time.Time
 	db    WritableDB
 }
@@ -103,15 +94,15 @@ func (c *udpLikeConnDB) Close() error {
 // address containing a domain name will fail. This QUICDialer will
 // save any event into the WritableDB. Any QUICConn created by it will
 // likewise save any event into the WritableDB.
-func (mx *Measurer) NewQUICDialerWithoutResolver(db WritableDB, logger Logger) QUICDialer {
+func (mx *Measurer) NewQUICDialerWithoutResolver(db WritableDB, logger model.Logger) model.QUICDialer {
 	return &quicDialerDB{db: db, logger: logger, begin: mx.Begin}
 }
 
 type quicDialerDB struct {
-	netxlite.QUICDialer
+	model.QUICDialer
 	begin  time.Time
 	db     WritableDB
-	logger Logger
+	logger model.Logger
 }
 
 func (qh *quicDialerDB) DialContext(ctx context.Context, network, address string,