openlayers-directive is an angular module. When you are creating your demo app module, you are including a reference to the openlayers module.
So if you wanted to use other modules in your demo app module you would also include them here, where you are declaring your module for the first time.
var app = angular.module('demoapp', ['openlayers-directive', 'anotherModule', 'yetAnotherModule']);
In your code you can then pass in any services from these modules by simply including them as parameters.
So if you have a demoController you could pass in a service from one of the included modules and use it.
angular.module('demoApp').controller('demoContoller', function($scope, anotherModuleService)
$scope.someFunctionFiredFromController = function()
//I have access to this service because the module it
//belongs to was referenced by the demoApp module, and the
//service was injected into the controller