How can I search all plugins for composer’s vendor/autoload.php?

by

Autoloader setup: 

"autoload"   : {
   "files"   : ["functions.php"]
}

Once you have the autoloader setup properly, you just need to include that one file, like this:

require_once( 'vendor/autoload.php' );

Full tutorial here – https://torquemag.io/2014/11/improving-wordpress-plugin-development-composer/