fix(netxlite): clearly document quirk and make code robust (#468)

This quirk really saddens me. It's a piece of tech debt we're
carrying over from the original netx implementation.

We cannot remove it _until_ we have legacy netx code around.

The second best thing we can do is to clearly move this code in
a place where it's clear it's a quirk and write and use some extra
code that makes sure the quirk's assumptions are always met.

Sigh.

See https://github.com/ooni/probe/issues/1591

internal/netxlite/legacy.go

@@ -2,44 +2,9 @@ package netxlite
 
 import (
 	"context"
-	"errors"
 	"net"
-	"strings"
-
-	"github.com/ooni/probe-cli/v3/internal/errorsx"
 )
 
-// reduceErrors finds a known error in a list of errors since
-// it's probably most relevant.
-//
-// Deprecation warning
-//
-// Albeit still used, this function is now DEPRECATED.
-//
-// In perspective, we would like to transition to a scenario where
-// full dialing is NOT used for measurements and we return a multierror here.
-func reduceErrors(errorslist []error) error {
-	if len(errorslist) == 0 {
-		return nil
-	}
-	// If we have a known error, let's consider this the real error
-	// since it's probably most relevant. Otherwise let's return the
-	// first considering that (1) local resolvers likely will give
-	// us IPv4 first and (2) also our resolver does that. So, in case
-	// the user has no IPv6 connectivity, an IPv6 error is going to
-	// appear later in the list of errors.
-	for _, err := range errorslist {
-		var wrapper *errorsx.ErrWrapper
-		if errors.As(err, &wrapper) && !strings.HasPrefix(
-			err.Error(), "unknown_failure",
-		) {
-			return err
-		}
-	}
-	// TODO(bassosimone): handle this case in a better way
-	return errorslist[0]
-}
-
 // These vars export internal names to legacy ooni/probe-cli code.
 var (
 	DefaultDialer        = defaultDialer