How do I call a method in another module?

If your module (the caller) lists the module that you are calling (the callee) as a dependency in its description.yaml file (see Reference of properties in a Module's description.yaml file) then the caller can call the callee's methods in its php code:

class caller_module extends module_base_class {
    public function showSlot() {
        callee_module:doSomething();
    }
}

If the caller does not have the callee listed as a dependancy, the caller must first use the inc() function to try and include the callee:

class caller_module extends module_base_class {
    public function showSlot() {
        if (inc('callee_module')) {
            callee_module:doSomething();
        } else {
            //Do something else...
        }
    }
}

The caller should only try to call the callee if the inc() function returned true.