I have found the way to manually install package and auto load package classes without composer command,
By just adding a reference in \vendor\composer\autoload_psr4.php
'Omnipay\\MyPackage\\' => array($vendorDir . '/omnipay/mypackage/src')
This should work automatically now when the vendor\autoloader.php is called.
and one more thing need to know that, i have to pass some default data variables to blade template (extension/igniter/payregister/mycustomGeteway.php) can i use index() method to pass the data to /thems/my-thems/_partials/payregister/mycustomGeteway.blade.php file!?