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/testusing.bash

@@ -0,0 +1,23 @@
+#!/bin/bash
+
+#
+# This script simulates a user creating a new project that depends
+# on github.com/ooni/probe-engine@GITHUB_SHA.
+#
+
+set -ex
+mkdir -p /tmp/example.org/x
+cd /tmp/example.org/x
+go mod init example.org/x
+cat > main.go << EOF
+package main
+
+import "github.com/ooni/probe-engine/libminiooni"
+
+func main() {
+    libminiooni.Main()
+}
+EOF
+go get -v github.com/ooni/probe-engine@$GITHUB_SHA
+go build -v .
+./x --yes -OTunnel=psiphon -ni https://www.example.com urlgetter