Load external php library in custom module
You cannot require classes with require or library hooks. Nor should you be editing external packages.

You are supposed to require external php packages with Composer. Composer will update the autoloader. Then in your class, you can say:

use ExternalPackage\namespace\Foo;

In the header and Foo class is then usable. Furthermore, you can inject it to the constructor with a little more work.

I would advise using SoapClient instead of nusoap as that is very old.

Regarding SoapClient, here is something I solved recently injecting it proper to my class, because that's the same road you're heading down.

Dependency inject a class with values from a config object?