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/experiment/webconnectivity/endpoints.go

@@ -0,0 +1,77 @@
+package webconnectivity
+
+import (
+	"net"
+	"net/url"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/internal/runtimex"
+)
+
+// EndpointInfo describes a TCP/TLS endpoint.
+type EndpointInfo struct {
+	String       string // String representation
+	URLGetterURL string // URL for urlgetter
+}
+
+// EndpointsList is a list of EndpointInfo
+type EndpointsList []EndpointInfo
+
+// Endpoints returns a list of endpoints for TCP connect
+func (el EndpointsList) Endpoints() (out []string) {
+	out = []string{}
+	for _, ei := range el {
+		out = append(out, ei.String)
+	}
+	return
+}
+
+// URLs returns a list of URLs for TCP urlgetter
+func (el EndpointsList) URLs() (out []string) {
+	out = []string{}
+	for _, ei := range el {
+		out = append(out, ei.URLGetterURL)
+	}
+	return
+}
+
+// NewEndpoints creates a list of TCP/TLS endpoints to test from the
+// target URL and the list of resolved IP addresses.
+func NewEndpoints(URL *url.URL, addrs []string) (out EndpointsList) {
+	out = EndpointsList{}
+	port := NewEndpointPort(URL)
+	for _, addr := range addrs {
+		endpoint := net.JoinHostPort(addr, port.Port)
+		out = append(out, EndpointInfo{
+			String:       endpoint,
+			URLGetterURL: (&url.URL{Scheme: port.URLGetterScheme, Host: endpoint}).String(),
+		})
+	}
+	return
+}
+
+// EndpointPort is the port to be used by a TCP/TLS endpoint.
+type EndpointPort struct {
+	URLGetterScheme string
+	Port            string
+}
+
+// NewEndpointPort creates an EndpointPort from the given URL. This function
+// panic if the scheme is not `http` or `https` as well as if the host is not
+// valid. The latter should not happen if you used url.Parse.
+func NewEndpointPort(URL *url.URL) (out EndpointPort) {
+	if URL.Scheme != "http" && URL.Scheme != "https" {
+		panic("passed an unexpected scheme")
+	}
+	switch URL.Scheme {
+	case "http":
+		out.URLGetterScheme, out.Port = "tcpconnect", "80"
+	case "https":
+		out.URLGetterScheme, out.Port = "tlshandshake", "443"
+	}
+	if URL.Host != URL.Hostname() {
+		_, port, err := net.SplitHostPort(URL.Host)
+		runtimex.PanicOnError(err, "SplitHostPort should not fail here")
+		out.Port = port
+	}
+	return
+}