refactor(oohelperd): flatten package hierarchy (#834)

In https://github.com/ooni/probe-cli/pull/832's initial diff, I
mentioned it would be cool to flatten oohelperd's hier.

I'm doing this now, and just for the master branch.

This diff is mostly a mechanical refactoring with very light
and apparently rather safe manual changes.

internal/cmd/oohelperd/handler.go

@@ -0,0 +1,67 @@
+package main
+
+//
+// HTTP handler
+//
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+
+	"github.com/ooni/probe-cli/v3/internal/model"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
+	"github.com/ooni/probe-cli/v3/internal/runtimex"
+	"github.com/ooni/probe-cli/v3/internal/version"
+)
+
+// handler implements the Web Connectivity test helper HTTP API.
+type handler struct {
+	// MaxAcceptableBody is the MANDATORY maximum acceptable response body.
+	MaxAcceptableBody int64
+
+	// NewClient is the MANDATORY factory to create a new HTTPClient.
+	NewClient func() model.HTTPClient
+
+	// NewDialer is the MANDATORY factory to create a new Dialer.
+	NewDialer func() model.Dialer
+
+	// NewResolver is the MANDATORY factory for creating a new resolver.
+	NewResolver func() model.Resolver
+}
+
+var _ http.Handler = &handler{}
+
+// ServeHTTP implements http.Handler.ServeHTTP.
+func (h *handler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
+	w.Header().Add("Server", fmt.Sprintf(
+		"oohelperd/%s ooniprobe-engine/%s", version.Version, version.Version,
+	))
+	if req.Method != "POST" {
+		w.WriteHeader(400)
+		return
+	}
+	reader := &io.LimitedReader{R: req.Body, N: h.MaxAcceptableBody}
+	data, err := netxlite.ReadAllContext(req.Context(), reader)
+	if err != nil {
+		w.WriteHeader(400)
+		return
+	}
+	var creq ctrlRequest
+	if err := json.Unmarshal(data, &creq); err != nil {
+		w.WriteHeader(400)
+		return
+	}
+	cresp, err := measure(req.Context(), h, &creq)
+	if err != nil {
+		w.WriteHeader(400)
+		return
+	}
+	// We assume that the following call cannot fail because it's a
+	// clearly-serializable data structure.
+	data, err = json.Marshal(cresp)
+	runtimex.PanicOnError(err, "json.Marshal failed")
+	w.Header().Add("Content-Type", "application/json")
+	w.Write(data)
+}