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/experiment/tlstool/internal/internal.go

@@ -0,0 +1,57 @@
+// Package internal contains the implementation of tlstool.
+package internal
+
+import (
+	"time"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/netx"
+)
+
+// DialerConfig contains the config for creating a dialer
+type DialerConfig struct {
+	Dialer netx.Dialer
+	Delay  time.Duration
+	SNI    string
+}
+
+// NewSNISplitterDialer creates a new dialer that splits
+// outgoing messages such that the SNI should end up being
+// splitted into different TCP segments.
+func NewSNISplitterDialer(config DialerConfig) Dialer {
+	return Dialer{
+		Dialer: config.Dialer,
+		Delay:  config.Delay,
+		Splitter: func(b []byte) [][]byte {
+			return SNISplitter(b, []byte(config.SNI))
+		},
+	}
+}
+
+// NewThriceSplitterDialer creates a new dialer that splits
+// outgoing messages in three parts according to the circumvention
+// technique described by Kevin Boch in the Internet Measurement
+// Village 2020 <https://youtu.be/ksojSRFLbBM?t=1140>.
+func NewThriceSplitterDialer(config DialerConfig) Dialer {
+	return Dialer{
+		Dialer:   config.Dialer,
+		Delay:    config.Delay,
+		Splitter: Splitter84rest,
+	}
+}
+
+// NewRandomSplitterDialer creates a new dialer that splits
+// the SNI like the fixed splitting schema used by outline. See
+// github.com/Jigsaw-Code/outline-go-tun2socks.
+func NewRandomSplitterDialer(config DialerConfig) Dialer {
+	return Dialer{
+		Dialer:   config.Dialer,
+		Delay:    config.Delay,
+		Splitter: Splitter3264rand,
+	}
+}
+
+// NewVanillaDialer creates a new vanilla dialer that does
+// nothing and is used to establish a baseline.
+func NewVanillaDialer(config DialerConfig) Dialer {
+	return Dialer{Dialer: config.Dialer}
+}