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/tls_test.go

@@ -16,7 +16,6 @@ import (
 
 	"github.com/apex/log"
 	"github.com/google/go-cmp/cmp"
-	"github.com/ooni/probe-cli/v3/internal/netxlite/errorsx"
 	"github.com/ooni/probe-cli/v3/internal/netxlite/mocks"
 )
 
@@ -544,7 +543,7 @@ func TestTLSHandshakerErrWrapper(t *testing.T) {
 			}
 			ctx := context.Background()
 			conn, _, err := th.Handshake(ctx, &mocks.Conn{}, &tls.Config{})
-			if err == nil || err.Error() != errorsx.FailureEOFError {
+			if err == nil || err.Error() != FailureEOFError {
 				t.Fatal("unexpected err", err)
 			}
 			if conn != nil {