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/main_test.go

@@ -0,0 +1,15 @@
+package main
+
+import (
+	"testing"
+)
+
+func TestSmoke(t *testing.T) {
+	// Just check whether we can start and then tear down the server, so
+	// we have coverage of this code and when we see that some lines aren't
+	// covered we know these are genuine places where we're not testing
+	// the code rather than just places like this simple main.
+	go testableMain()
+	srvcancel()  // kills the listener
+	srvwg.Wait() // joined
+}