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/geolocate/iplookup_test.go

@@ -0,0 +1,56 @@
+package geolocate
+
+import (
+	"context"
+	"errors"
+	"net"
+	"net/http"
+	"testing"
+
+	"github.com/apex/log"
+)
+
+func TestIPLookupGood(t *testing.T) {
+	ip, err := (ipLookupClient{
+		HTTPClient: http.DefaultClient,
+		Logger:     log.Log,
+		UserAgent:  "ooniprobe-engine/0.1.0",
+	}).LookupProbeIP(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if net.ParseIP(ip) == nil {
+		t.Fatal("not an IP address")
+	}
+}
+
+func TestIPLookupAllFailed(t *testing.T) {
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel() // immediately cancel to cause Do() to fail
+	ip, err := (ipLookupClient{
+		HTTPClient: http.DefaultClient,
+		Logger:     log.Log,
+		UserAgent:  "ooniprobe-engine/0.1.0",
+	}).LookupProbeIP(ctx)
+	if !errors.Is(err, context.Canceled) {
+		t.Fatal("expected an error here")
+	}
+	if ip != DefaultProbeIP {
+		t.Fatal("expected the default IP here")
+	}
+}
+
+func TestIPLookupInvalidIP(t *testing.T) {
+	ctx := context.Background()
+	ip, err := (ipLookupClient{
+		HTTPClient: http.DefaultClient,
+		Logger:     log.Log,
+		UserAgent:  "ooniprobe-engine/0.1.0",
+	}).doWithCustomFunc(ctx, invalidIPLookup)
+	if !errors.Is(err, ErrInvalidIPAddress) {
+		t.Fatal("expected an error here")
+	}
+	if ip != DefaultProbeIP {
+		t.Fatal("expected the default IP here")
+	}
+}