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/experiment/webconnectivity/dnsanalysis_test.go

@@ -7,11 +7,11 @@ import (
 
 	"github.com/google/go-cmp/cmp"
 	"github.com/ooni/probe-cli/v3/internal/engine/experiment/webconnectivity"
-	"github.com/ooni/probe-cli/v3/internal/netxlite/errorsx"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
 
 func TestDNSAnalysis(t *testing.T) {
-	measurementFailure := errorsx.FailureDNSNXDOMAINError
+	measurementFailure := netxlite.FailureDNSNXDOMAINError
 	controlFailure := webconnectivity.DNSNameError
 	eofFailure := io.EOF.Error()
 	type args struct {