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/internal/platform/platform.go

@@ -0,0 +1,46 @@
+// Package platform returns the platform name. The name returned here
+// is compatible with the names returned by Measurement Kit.
+package platform
+
+import "runtime"
+
+// Name returns the platform name. The returned value is one of:
+//
+// 1. "android"
+// 2. "ios"
+// 3. "linux"
+// 5. "macos"
+// 4. "windows"
+// 5. "unknown"
+//
+// The android, ios, linux, macos, windows, and unknown strings are
+// also returned by Measurement Kit. As a known bug, the detection of
+// darwin-based systems relies on the architecture, when CGO support
+// has been disabled. In such case, the code will return "ios" when
+// using arm{,64} and "macos" when using x86{,_64}.
+func Name() string {
+	if name := cgoname(); name != "unknown" {
+		return name
+	}
+	return puregoname(runtime.GOOS, runtime.GOARCH)
+}
+
+func puregoname(goos, goarch string) string {
+	switch goos {
+	case "android", "linux", "windows":
+		return goos
+	case "darwin":
+		return detectDarwin(goarch)
+	}
+	return "unknown"
+}
+
+func detectDarwin(goarch string) string {
+	switch goarch {
+	case "386", "amd64":
+		return "macos"
+	case "arm", "arm64":
+		return "ios"
+	}
+	return "unknown"
+}