Simplify how plugin URL and path are retrieved

[tfirdaus]

@troychaplin This Pull Request is a suggestion on how we could simplify retrieving the plugin URL and directory path.

While the Plugin_Paths class is nice, I think we could just define the __FILE__ and __DIR__ constants once in the main plugin file and then reuse them across the plugin.

I've been using this approach in my own boilerplate and other plugins, and it works just as well. Theoretically, it should also be a bit faster since we avoid repeatedly calling functions like plugin_dir_path() or dirname() whenever we need the plugin's directory.

The PR also adds a namespace in the plugin's main file, which effectively scopes these constants. Even if the constants don't have sort of prefix on the name, they'd need to be referenced with their fully qualified name when they're used in other files with different namespaces. I think that could serve as a nice demonstration of how namespaces work in practice within the tutorial.

There are more rooms for improvements, such as that I think we can ignore the vendor directory from source, especially since we don't have any dependencies that we are going to use in production. The current dependencies are only used in the development.

I also think that Plugin_Paths can be a function since the class contains just static methods, and it's a good opportunity for the tutorial to show how to use namespace for a function, and how to autoload a function with Composer.

But I think for the moment, I'd just focus on this very specific things :)

[tfirdaus]

I added the plugin namespace in the main file, so it's no longer necessary to reference the class with the full namespace.

Also, the Plugin_Paths file has been removed and replaced with constants when referring to the plugin directory or file. If we decide to keep the class, I don't think it needs to be included in the list or instantiated, since it only contains static methods since Composer will still autoload it when those methods are called.