enabled sftp private key support (instead of password)

this is the code to make #9182 work
It is a combination of
a) You are not creating an instance of Net_SFTP_Stream (because obviously you don't have to), so this file is not loaded by the autoloader.
b) The Net/SFTP/Stream.php file has a side effect (which is not a good idea) that registers "sftp://" as a stream.

phpseclib should make this explicit, possibly just using Net_SFTP_Stream::register() which will get rid of the side effect and allows proper autoloading.