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.go

@@ -0,0 +1,113 @@
+package geolocate
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"math/rand"
+	"net"
+	"net/http"
+	"sync"
+	"time"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/internal/multierror"
+)
+
+var (
+	// ErrAllIPLookuppersFailed indicates that we failed with looking
+	// up the probe IP for with all the lookuppers that we tried.
+	ErrAllIPLookuppersFailed = errors.New("all IP lookuppers failed")
+
+	// ErrInvalidIPAddress indicates that the code returned to us a
+	// string that actually isn't a valid IP address.
+	ErrInvalidIPAddress = errors.New("lookupper did not return a valid IP")
+)
+
+type lookupFunc func(
+	ctx context.Context, client *http.Client,
+	logger Logger, userAgent string,
+) (string, error)
+
+type method struct {
+	name string
+	fn   lookupFunc
+}
+
+var (
+	methods = []method{
+		{
+			name: "avast",
+			fn:   avastIPLookup,
+		},
+		{
+			name: "ipconfig",
+			fn:   ipConfigIPLookup,
+		},
+		{
+			name: "ipinfo",
+			fn:   ipInfoIPLookup,
+		},
+		{
+			name: "stun_ekiga",
+			fn:   stunEkigaIPLookup,
+		},
+		{
+			name: "stun_google",
+			fn:   stunGoogleIPLookup,
+		},
+		{
+			name: "ubuntu",
+			fn:   ubuntuIPLookup,
+		},
+	}
+
+	once sync.Once
+)
+
+type ipLookupClient struct {
+	// HTTPClient is the HTTP client to use
+	HTTPClient *http.Client
+
+	// Logger is the logger to use
+	Logger Logger
+
+	// UserAgent is the user agent to use
+	UserAgent string
+}
+
+func makeSlice() []method {
+	r := rand.New(rand.NewSource(time.Now().UnixNano()))
+	ret := make([]method, len(methods))
+	perm := r.Perm(len(methods))
+	for idx, randIdx := range perm {
+		ret[idx] = methods[randIdx]
+	}
+	return ret
+}
+
+func (c ipLookupClient) doWithCustomFunc(
+	ctx context.Context, fn lookupFunc,
+) (string, error) {
+	ip, err := fn(ctx, c.HTTPClient, c.Logger, c.UserAgent)
+	if err != nil {
+		return DefaultProbeIP, err
+	}
+	if net.ParseIP(ip) == nil {
+		return DefaultProbeIP, fmt.Errorf("%w: %s", ErrInvalidIPAddress, ip)
+	}
+	c.Logger.Debugf("iplookup: IP: %s", ip)
+	return ip, nil
+}
+
+func (c ipLookupClient) LookupProbeIP(ctx context.Context) (string, error) {
+	union := multierror.New(ErrAllIPLookuppersFailed)
+	for _, method := range makeSlice() {
+		c.Logger.Debugf("iplookup: using %s", method.name)
+		ip, err := c.doWithCustomFunc(ctx, method.fn)
+		if err == nil {
+			return ip, nil
+		}
+		union.Add(err)
+	}
+	return DefaultProbeIP, union
+}