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/legacy/netx/oldhttptransport/bodytracer.go

@@ -0,0 +1,82 @@
+package oldhttptransport
+
+import (
+	"io"
+	"net/http"
+	"time"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/legacy/netx/modelx"
+	"github.com/ooni/probe-cli/v3/internal/engine/legacy/netx/transactionid"
+)
+
+// BodyTracer performs single HTTP transactions and emits
+// measurement events as they happen.
+type BodyTracer struct {
+	Transport http.RoundTripper
+}
+
+// NewBodyTracer creates a new Transport.
+func NewBodyTracer(roundTripper http.RoundTripper) *BodyTracer {
+	return &BodyTracer{Transport: roundTripper}
+}
+
+// RoundTrip executes a single HTTP transaction, returning
+// a Response for the provided Request.
+func (t *BodyTracer) RoundTrip(req *http.Request) (resp *http.Response, err error) {
+	resp, err = t.Transport.RoundTrip(req)
+	if err != nil {
+		return
+	}
+	// "The http Client and Transport guarantee that Body is always
+	//  non-nil, even on responses without a body or responses with
+	//  a zero-length body." (from the docs)
+	resp.Body = &bodyWrapper{
+		ReadCloser: resp.Body,
+		root:       modelx.ContextMeasurementRootOrDefault(req.Context()),
+		tid:        transactionid.ContextTransactionID(req.Context()),
+	}
+	return
+}
+
+// CloseIdleConnections closes the idle connections.
+func (t *BodyTracer) CloseIdleConnections() {
+	// Adapted from net/http code
+	type closeIdler interface {
+		CloseIdleConnections()
+	}
+	if tr, ok := t.Transport.(closeIdler); ok {
+		tr.CloseIdleConnections()
+	}
+}
+
+type bodyWrapper struct {
+	io.ReadCloser
+	root *modelx.MeasurementRoot
+	tid  int64
+}
+
+func (bw *bodyWrapper) Read(b []byte) (n int, err error) {
+	n, err = bw.ReadCloser.Read(b)
+	bw.root.Handler.OnMeasurement(modelx.Measurement{
+		HTTPResponseBodyPart: &modelx.HTTPResponseBodyPartEvent{
+			// "Read reads up to len(p) bytes into p. It returns the number of
+			// bytes read (0 <= n <= len(p)) and any error encountered."
+			Data:                   b[:n],
+			Error:                  err,
+			DurationSinceBeginning: time.Now().Sub(bw.root.Beginning),
+			TransactionID:          bw.tid,
+		},
+	})
+	return
+}
+
+func (bw *bodyWrapper) Close() (err error) {
+	err = bw.ReadCloser.Close()
+	bw.root.Handler.OnMeasurement(modelx.Measurement{
+		HTTPResponseDone: &modelx.HTTPResponseDoneEvent{
+			DurationSinceBeginning: time.Now().Sub(bw.root.Beginning),
+			TransactionID:          bw.tid,
+		},
+	})
+	return
+}