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/internal/webconnectivity/webconnectivity.go

@@ -1,54 +0,0 @@
-package webconnectivity
-
-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 int64
-	NewClient         func() model.HTTPClient
-	NewDialer         func() model.Dialer
-	NewResolver       func() model.Resolver
-}
-
-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
-	}
-	measureConfig := MeasureConfig(h)
-	cresp, err := Measure(req.Context(), measureConfig, &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)
-}