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/conn.go

@@ -6,8 +6,8 @@ package bytecounter
 
 import "net"
 
-// Conn wraps a network connection and counts bytes.
-type Conn struct {
+// wrappedConn wraps a network connection and counts bytes.
+type wrappedConn struct {
 	// net.Conn is the underlying net.Conn.
 	net.Conn
 
@@ -16,28 +16,28 @@ type Conn struct {
 }
 
 // Read implements net.Conn.Read.
-func (c *Conn) Read(p []byte) (int, error) {
+func (c *wrappedConn) Read(p []byte) (int, error) {
 	count, err := c.Conn.Read(p)
 	c.Counter.CountBytesReceived(count)
 	return count, err
 }
 
 // Write implements net.Conn.Write.
-func (c *Conn) Write(p []byte) (int, error) {
+func (c *wrappedConn) Write(p []byte) (int, error) {
 	count, err := c.Conn.Write(p)
 	c.Counter.CountBytesSent(count)
 	return count, err
 }
 
-// Wrap returns a new conn that uses the given counter.
-func Wrap(conn net.Conn, counter *Counter) net.Conn {
-	return &Conn{Conn: conn, Counter: counter}
+// WrapConn returns a new conn that uses the given counter.
+func WrapConn(conn net.Conn, counter *Counter) net.Conn {
+	return &wrappedConn{Conn: conn, Counter: counter}
 }
 
-// MaybeWrap is like wrap if counter is not nil, otherwise it's a no-op.
-func MaybeWrap(conn net.Conn, counter *Counter) net.Conn {
+// MaybeWrapConn is like wrap if counter is not nil, otherwise it's a no-op.
+func MaybeWrapConn(conn net.Conn, counter *Counter) net.Conn {
 	if counter == nil {
 		return conn
 	}
-	return Wrap(conn, counter)
+	return WrapConn(conn, counter)
 }