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/engine/netx/resolver/bogon_test.go

@@ -6,7 +6,7 @@ import (
 	"testing"
 
 	"github.com/ooni/probe-cli/v3/internal/engine/netx/resolver"
-	"github.com/ooni/probe-cli/v3/internal/netxlite/errorsx"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
 
 func TestResolverIsBogon(t *testing.T) {
@@ -29,7 +29,7 @@ func TestBogonAwareResolverWithBogon(t *testing.T) {
 		Resolver: resolver.NewFakeResolverWithResult([]string{"127.0.0.1"}),
 	}
 	addrs, err := r.LookupHost(context.Background(), "dns.google.com")
-	if !errors.Is(err, errorsx.ErrDNSBogon) {
+	if !errors.Is(err, netxlite.ErrDNSBogon) {
 		t.Fatal("not the error we expected")
 	}
 	if len(addrs) > 0 {