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/ptx/ptx.go

@@ -163,8 +163,8 @@ func (lst *Listener) handleSocksConn(ctx context.Context, socksConn ptxSocksConn
 	// We _must_ wrap the ptConn. Wrapping the socks conn leads us to
 	// count the sent bytes as received and the received bytes as sent:
 	// bytes flow in the opposite direction there for the socks conn.
-	ptConn = bytecounter.MaybeWrap(ptConn, lst.SessionByteCounter)
-	ptConn = bytecounter.MaybeWrap(ptConn, lst.ExperimentByteCounter)
+	ptConn = bytecounter.MaybeWrapConn(ptConn, lst.SessionByteCounter)
+	ptConn = bytecounter.MaybeWrapConn(ptConn, lst.ExperimentByteCounter)
 	lst.forwardWithContext(ctx, socksConn, ptConn) // transfer ownership
 	return nil                                     // used for testing
 }