chore: merge probe-engine into probe-cli (#201)

This is how I did it:

1. `git clone https://github.com/ooni/probe-engine internal/engine`

2. ```
(cd internal/engine && git describe --tags)
v0.23.0
```

3. `nvim go.mod` (merging `go.mod` with `internal/engine/go.mod`

4. `rm -rf internal/.git internal/engine/go.{mod,sum}`

5. `git add internal/engine`

6. `find . -type f -name \*.go -exec sed -i 's@/ooni/probe-engine@/ooni/probe-cli/v3/internal/engine@g' {} \;`

7. `go build ./...` (passes)

8. `go test -race ./...` (temporary failure on RiseupVPN)

9. `go mod tidy`

10. this commit message

Once this piece of work is done, we can build a new version of `ooniprobe` that
is using `internal/engine` directly. We need to do more work to ensure all the
other functionality in `probe-engine` (e.g. making mobile packages) are still WAI.

Part of https://github.com/ooni/probe/issues/1335

internal/engine/netx/dialer/dns.go

@@ -0,0 +1,73 @@
+package dialer
+
+import (
+	"context"
+	"errors"
+	"net"
+	"strings"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/legacy/netx/dialid"
+	"github.com/ooni/probe-cli/v3/internal/engine/netx/errorx"
+)
+
+// DNSDialer is a dialer that uses the configured Resolver to resolver a
+// domain name to IP addresses, and the configured Dialer to connect.
+type DNSDialer struct {
+	Dialer
+	Resolver Resolver
+}
+
+// DialContext implements Dialer.DialContext.
+func (d DNSDialer) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
+	onlyhost, onlyport, err := net.SplitHostPort(address)
+	if err != nil {
+		return nil, err
+	}
+	ctx = dialid.WithDialID(ctx) // important to create before lookupHost
+	var addrs []string
+	addrs, err = d.LookupHost(ctx, onlyhost)
+	if err != nil {
+		return nil, err
+	}
+	var errorslist []error
+	for _, addr := range addrs {
+		target := net.JoinHostPort(addr, onlyport)
+		conn, err := d.Dialer.DialContext(ctx, network, target)
+		if err == nil {
+			return conn, nil
+		}
+		errorslist = append(errorslist, err)
+	}
+	return nil, ReduceErrors(errorslist)
+}
+
+// ReduceErrors finds a known error in a list of errors since it's probably most relevant
+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 *errorx.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]
+}
+
+// LookupHost implements Resolver.LookupHost
+func (d DNSDialer) LookupHost(ctx context.Context, hostname string) ([]string, error) {
+	if net.ParseIP(hostname) != nil {
+		return []string{hostname}, nil
+	}
+	return d.Resolver.LookupHost(ctx, hostname)
+}