Angular App works when all components are in one file, but throws 'nomod' error when components are in separate files

I'm working through this tutorial on creating a single-page MEAN stack todo app. I'm on this step, specifically. The tutorial covers modularization of code, and while I was able to separate my backend code (Express, Mongo, etc.) into modules successfully, when I separate my angular.js code, the todo app ceases to function. The specific error which is thrown to the console is "Uncaught Error: [$injector:modulerr]." The specific error is "nomod" (i.e. the module "simpleTodo" is failing to load.) I'd appreciate any help.

Code as one file (core.js):

var simpleTodo = angular.module('simpleTodo', []);

simpleTodo.controller('mainController', ['$scope', '$http', function($scope, $http) {
$scope.formData = {};

$http.get('/api/todos')
  .success(function(data) {
        $scope.todos = data;
    })
    .error(function(data) {
        console.log('Error: ' + data);
    });

$scope.createTodo = function() {
    $http.post('/api/todos', $scope.formData)
      .success(function(data) {
            $scope.formData = {};
            $scope.todos = data;
        })
        .error(function(data) {
            console.log('Error: ' + data);
        });
};

$scope.deleteTodo = function(id) {
    $http.delete('/api/todos/' + id)
      .success(function(data) {
            $scope.todos = data;
        })
        .error(function(data) {
            console.log('Error: ' + data);
        });
    };
}]);

Code in modules:

New core.js:

var simpleTodo = angular.module('simpleTodo',['todoController', 'todoService']);

Create/Delete Todo Service (todos.js):

angular.module('todoService', [])
  .factory('Todos', ['$http', function($http) {
      return {
            get: function() {
                return $http.get('/api/todos');
            },
            create: function(todoData) {
                return $http.post('/api/todos', todoData);
            },
            delete: function(id) {
                return $http.delete('/api/todos/' + id);
            }
        }
}]);

Controller file (main.js)

angular.module('todoController', [])
  .controller('mainController', ['$scope', '$http', 'Todos', function($scope, $http, Todos) {
    $scope.formData = {};

    Todos.get()
      .success(function(data) {
            $scope.todos = data;
        });

    $scope.createTodo = function() {
        if ($scope.formData !== null) {
            Todos.create($scope.formData)
              .success(function(data) {
                    $scope.formData = {};
                    $scope.todos = data;
                });
        }
    };

    $scope.deleteTodo = function(id) {
        Todos.delete(id)
          .success(function(data) {
                $scope.todos = data;
            });
    };
}]);

Order of script loading on index.html:

<script src="http://ift.tt/1yCEpkO"></script>
<script src="http://ift.tt/1MwNz7a"></script>
<script src="js/controllers/main.js"></script>
<script src="js/services/todos.js"</script>
<script src="js/core.js"></script>

Thanks!

New info: After following floribon's advice, I get the same error, except instead of "simpleTodo" failing to load, it is "todoController" that cannot be loaded. I appreciate his advice but it still isn't working. :( Here's a github repo with his changes implemented, if you want to see: http://ift.tt/1MUBvfY