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/model/experiment.go

@@ -0,0 +1,75 @@
+package model
+
+import (
+	"context"
+	"net/http"
+)
+
+// ExperimentOrchestraClient is the experiment's view of
+// a client for querying the OONI orchestra API.
+type ExperimentOrchestraClient interface {
+	FetchPsiphonConfig(ctx context.Context) ([]byte, error)
+	FetchTorTargets(ctx context.Context, cc string) (map[string]TorTarget, error)
+	FetchURLList(ctx context.Context, config URLListConfig) ([]URLInfo, error)
+}
+
+// ExperimentSession is the experiment's view of a session.
+type ExperimentSession interface {
+	ASNDatabasePath() string
+	GetTestHelpersByName(name string) ([]Service, bool)
+	DefaultHTTPClient() *http.Client
+	Logger() Logger
+	NewOrchestraClient(ctx context.Context) (ExperimentOrchestraClient, error)
+	ProbeCC() string
+	ResolverIP() string
+	TempDir() string
+	TorArgs() []string
+	TorBinary() string
+	UserAgent() string
+}
+
+// ExperimentCallbacks contains experiment event-handling callbacks
+type ExperimentCallbacks interface {
+	// OnProgress provides information about an experiment progress.
+	OnProgress(percentage float64, message string)
+}
+
+// PrinterCallbacks is the default event handler
+type PrinterCallbacks struct {
+	Logger
+}
+
+// NewPrinterCallbacks returns a new default callback handler
+func NewPrinterCallbacks(logger Logger) PrinterCallbacks {
+	return PrinterCallbacks{Logger: logger}
+}
+
+// OnProgress provides information about an experiment progress.
+func (d PrinterCallbacks) OnProgress(percentage float64, message string) {
+	d.Logger.Infof("[%5.1f%%] %s", percentage*100, message)
+}
+
+// ExperimentMeasurer is the interface that allows to run a
+// measurement for a specific experiment.
+type ExperimentMeasurer interface {
+	// ExperimentName returns the experiment name.
+	ExperimentName() string
+
+	// ExperimentVersion returns the experiment version.
+	ExperimentVersion() string
+
+	// Run runs the experiment with the specified context, session,
+	// measurement, and experiment calbacks. This method should only
+	// return an error in case the experiment could not run (e.g.,
+	// a required input is missing). Otherwise, the code should just
+	// set the relevant OONI error inside of the measurmeent and
+	// return nil. This is important because the caller may not submit
+	// the measurement if this method returns an error.
+	Run(
+		ctx context.Context, sess ExperimentSession,
+		measurement *Measurement, callbacks ExperimentCallbacks,
+	) error
+
+	// GetSummaryKeys returns summary keys expected by ooni/probe-cli.
+	GetSummaryKeys(*Measurement) (interface{}, error)
+}