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/run/run.go

@@ -0,0 +1,79 @@
+// Package run contains code to run other experiments.
+//
+// This code is currently alpha.
+package run
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/experiment/dnscheck"
+	"github.com/ooni/probe-cli/v3/internal/engine/experiment/urlgetter"
+	"github.com/ooni/probe-cli/v3/internal/engine/model"
+)
+
+// Config contains settings.
+type Config struct{}
+
+// Measurer runs the measurement.
+type Measurer struct{}
+
+// ExperimentName implements ExperimentMeasurer.ExperimentName.
+func (Measurer) ExperimentName() string {
+	return "run"
+}
+
+// ExperimentVersion implements ExperimentMeasurer.ExperimentVersion.
+func (Measurer) ExperimentVersion() string {
+	return "0.2.0"
+}
+
+// StructuredInput contains structured input for this experiment.
+type StructuredInput struct {
+	// Annotations contains extra annotations to add to the
+	// final measurement.
+	Annotations map[string]string `json:"annotations"`
+
+	// DNSCheck contains settings for the dnscheck experiment.
+	DNSCheck dnscheck.Config `json:"dnscheck"`
+
+	// URLGetter contains settings for the urlgetter experiment.
+	URLGetter urlgetter.Config `json:"urlgetter"`
+
+	// Name is the name of the experiment to run.
+	Name string `json:"name"`
+
+	// Input is the input for this experiment.
+	Input string `json:"input"`
+}
+
+// Run implements ExperimentMeasurer.ExperimentVersion.
+func (Measurer) Run(
+	ctx context.Context, sess model.ExperimentSession,
+	measurement *model.Measurement, callbacks model.ExperimentCallbacks,
+) error {
+	var input StructuredInput
+	if err := json.Unmarshal([]byte(measurement.Input), &input); err != nil {
+		return err
+	}
+	exprun, found := table[input.Name]
+	if !found {
+		return fmt.Errorf("no such experiment: %s", input.Name)
+	}
+	measurement.AddAnnotations(input.Annotations)
+	return exprun.do(ctx, input, sess, measurement, callbacks)
+}
+
+// GetSummaryKeys implements ExperimentMeasurer.GetSummaryKeys
+func (Measurer) GetSummaryKeys(*model.Measurement) (interface{}, error) {
+	// TODO(bassosimone): we could extend this interface to call the
+	// specific GetSummaryKeys of the experiment we're running.
+	return dnscheck.SummaryKeys{IsAnomaly: false}, nil
+}
+
+// NewExperimentMeasurer creates a new model.ExperimentMeasurer
+// implementing the run experiment.
+func NewExperimentMeasurer(config Config) model.ExperimentMeasurer {
+	return Measurer{}
+}