refactor(netx): move construction logic outside package (#798)

For testability, replace most if-based construction logic with
calls to well-tested factories living in other packages.

While there, acknowledge that a bunch of types could now be private
and make them private, modifying the code to call the public
factories allowing to construct said types instead.

Part of https://github.com/ooni/probe/issues/2121

internal/bytecounter/context.go

@@ -38,7 +38,7 @@ func WithExperimentByteCounter(ctx context.Context, counter *Counter) context.Co
 // MaybeWrapWithContextByteCounters wraps a conn with the byte counters
 // that have previosuly been configured into a context.
 func MaybeWrapWithContextByteCounters(ctx context.Context, conn net.Conn) net.Conn {
-	conn = MaybeWrap(conn, ContextExperimentByteCounter(ctx))
-	conn = MaybeWrap(conn, ContextSessionByteCounter(ctx))
+	conn = MaybeWrapConn(conn, ContextExperimentByteCounter(ctx))
+	conn = MaybeWrapConn(conn, ContextSessionByteCounter(ctx))
 	return conn
 }