cleanup: remove unnecessary legacy interfaces (#656)

This diff addresses another point of https://github.com/ooni/probe/issues/1956:

> - [ ] observe that we're still using a bunch of private interfaces for common interfaces such as the `Dialer`, so we can get rid of these private interfaces and always use the ones in `model`, which allows us to remove a bunch of legacy wrappers

Additional cleanups may still be possible. The more I cleanup, the more I see
there's extra legacy code we can dispose of (which seems good?).

internal/engine/netx/resolver/chain.go

@@ -2,13 +2,15 @@ package resolver
 
 import (
 	"context"
+
+	"github.com/ooni/probe-cli/v3/internal/model"
 )
 
 // ChainResolver is a chain resolver. The primary resolver is used first and, if that
 // fails, we then attempt with the secondary resolver.
 type ChainResolver struct {
-	Primary   Resolver
-	Secondary Resolver
+	Primary   model.Resolver
+	Secondary model.Resolver
 }
 
 // LookupHost implements Resolver.LookupHost
@@ -30,4 +32,20 @@ func (c ChainResolver) Address() string {
 	return ""
 }
 
-var _ Resolver = ChainResolver{}
+// CloseIdleConnections implements Resolver.CloseIdleConnections.
+func (c ChainResolver) CloseIdleConnections() {
+	c.Primary.CloseIdleConnections()
+	c.Secondary.CloseIdleConnections()
+}
+
+// LookupHTTPS implements Resolver.LookupHTTPS
+func (c ChainResolver) LookupHTTPS(
+	ctx context.Context, domain string) (*model.HTTPSSvc, error) {
+	https, err := c.Primary.LookupHTTPS(ctx, domain)
+	if err != nil {
+		https, err = c.Secondary.LookupHTTPS(ctx, domain)
+	}
+	return https, err
+}
+
+var _ model.Resolver = ChainResolver{}