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/measurer.go

@@ -21,6 +21,7 @@ import (
 
 	"github.com/apex/log"
 	"github.com/lucas-clemente/quic-go"
+	"github.com/ooni/probe-cli/v3/internal/model"
 	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
 
@@ -34,7 +35,7 @@ type Measurer struct {
 	HTTPClient HTTPClient
 
 	// Logger is the MANDATORY logger to use.
-	Logger Logger
+	Logger model.Logger
 
 	// MeasureURLHelper is the OPTIONAL test helper to use when
 	// we're measuring using the MeasureURL function. If this field
@@ -45,7 +46,7 @@ type Measurer struct {
 	Resolvers []*ResolverInfo
 
 	// TLSHandshaker is the MANDATORY TLS handshaker.
-	TLSHandshaker netxlite.TLSHandshaker
+	TLSHandshaker model.TLSHandshaker
 }
 
 // NewMeasurerWithDefaultSettings creates a new Measurer
@@ -85,7 +86,7 @@ func (mx *Measurer) LookupHostSystem(ctx context.Context, domain string) *DNSMea
 
 // lookupHostForeign performs a LookupHost using a "foreign" resolver.
 func (mx *Measurer) lookupHostForeign(
-	ctx context.Context, domain string, r Resolver) *DNSMeasurement {
+	ctx context.Context, domain string, r model.Resolver) *DNSMeasurement {
 	const timeout = 4 * time.Second
 	ol := NewOperationLogger(mx.Logger, "LookupHost %s with %s", domain, r.Network())
 	ctx, cancel := context.WithTimeout(ctx, timeout)
@@ -158,7 +159,7 @@ func (mx *Measurer) LookupHTTPSSvcUDP(
 // lookupHTTPSSvcUDPForeign is like LookupHTTPSSvcUDP
 // except that it uses a "foreign" resolver.
 func (mx *Measurer) lookupHTTPSSvcUDPForeign(
-	ctx context.Context, domain string, r Resolver) *DNSMeasurement {
+	ctx context.Context, domain string, r model.Resolver) *DNSMeasurement {
 	const timeout = 4 * time.Second
 	ol := NewOperationLogger(mx.Logger, "LookupHTTPSvc %s with %s", domain, r.Address())
 	ctx, cancel := context.WithTimeout(ctx, timeout)
@@ -591,7 +592,7 @@ type ResolverInfo struct {
 
 	// ForeignResolver is only used when Network's
 	// value equals the ResolverForeign constant.
-	ForeignResolver Resolver
+	ForeignResolver model.Resolver
 }
 
 // LookupURLHostParallel performs an LookupHost-like operation for each