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/CLI/README.md

@@ -0,0 +1,3 @@
+# Directory github.com/ooni/probe-engine/CLI
+
+We use this directory for building CLI binaries (e.g. miniooni).

internal/engine/CLI/darwin/amd64/.gitignore

@@ -0,0 +1 @@
+/miniooni

internal/engine/CLI/linux/386/.gitignore

@@ -0,0 +1 @@
+/miniooni

internal/engine/CLI/linux/amd64/.gitignore

@@ -0,0 +1 @@
+/miniooni

internal/engine/CLI/linux/arm64/.gitignore

@@ -0,0 +1 @@
+/miniooni

internal/engine/CLI/linux/armv7/.gitignore

@@ -0,0 +1 @@
+/miniooni

internal/engine/CLI/windows/386/.gitignore

@@ -0,0 +1 @@
+/miniooni.exe

internal/engine/CLI/windows/amd64/.gitignore

@@ -0,0 +1 @@
+/miniooni.exe