refactor: merge dnsx and errorsx into netxlite (#517)

When preparing a tutorial for netxlite, I figured it is easier
to tell people "hey, this is the package you should use for all
low-level networking stuff" rather than introducing people to
a set of packages working together where some piece of functionality
is here and some other piece is there.

Part of https://github.com/ooni/probe/issues/1591

internal/netxlite/mocks/resolver.go

@@ -2,8 +2,6 @@ package mocks
 
 import (
 	"context"
-
-	"github.com/ooni/probe-cli/v3/internal/netxlite/dnsx/model"
 )
 
 // Resolver is a mockable Resolver.
@@ -35,9 +33,6 @@ func (r *Resolver) CloseIdleConnections() {
 	r.MockCloseIdleConnections()
 }
 
-// HTTPSSvc is an HTTPSSvc reply.
-type HTTPSSvc = model.HTTPSSvc
-
 // LookupHTTPS calls MockLookupHTTPS.
 func (r *Resolver) LookupHTTPS(ctx context.Context, domain string) (*HTTPSSvc, error) {
 	return r.MockLookupHTTPS(ctx, domain)