Composer - Setup autoload in new package - php

Question

I'm trying to make a composer package, but I'm struggling to setup the autoload to work in the project where I'm developing it.
I have two projects:
Project Foo (where I'm developing the package).
Project Bar (where I've installed the package: composer require myhandle/mypackage).
In Project Foo I (obviously) have to be able to use the package as well. But setting up the autoload in my own package is not working.
However... When I commit the changes to Github, update the package at Packagist and run composer update in Project Bar, then it works(?!).
And wierdly enough... If I do composer update from Project Foo (the project where it isn't working). So updating the package to it's current version (?), makes it start working.
So it must somehow be related to how I've setup the autoload.
A note:
I started the package by making a new (empty) folder in the vendor directory, and then build the package there. I figured it was smart, since it then would mirror how it would look had I composer required the package.
I don't know if this is bad practice, since Composer no longer 'are in control' of all files in the vendor directory.
There are tons of guides on how to make a composer package out there, - but non of them explains about a good way to structure the files in the project where the package is being developed.
Here's what I do to 'get the error' (in Project Foo):
Create new class file, such as: myhandle/mypackage/src/Test.php
Then I instantiate it like this: $test = new MyNamespace\MyPackageName\Test();
And then I get the error:
Fatal error: Uncaught Error: Class 'MyNamespace\MyPackageName\Test' not found
And this is what works in Project Bar (the very same code).
I can't find a guide on how to correctly setup autoload in the package I'm developing. I'm using this autoload file, that I found in another composer project. I've put it in the root of my project. It looks like this:
<?php
namespace MyNamespace\MyPackageName;
spl_autoload_register(function($cls) {
$cls = ltrim($cls, '\\');
if (strpos($cls, __NAMESPACE__) !== 0) {
return;
}
$classWithoutBaseNamespace = str_replace(__NAMESPACE__, '', $cls);
// Load files from 'src' directory based on their class name without
// the StoutLogic\AcfBuilder namespace.
$path = dirname(__FILE__).
DIRECTORY_SEPARATOR.
'src'.
str_replace('\\', DIRECTORY_SEPARATOR, $classWithoutBaseNamespace).
'.php';
require_once($path);
});
I can't find it in the Composer Documentation, how to set it up in a new project/package. However I can find a bazillions guides on how to use autoload.

Answer 1

As yivi and Daniel Protopopov pointed out:
Check the documentation at getcomposer.org regarding autoloading
Delete your custom autoloader definition, register your namespace in composer.json (hoping you follow PSR-4 already), run composer dump-autoload.
Last but not least, when- and wherever you need to use it, just include the
require __DIR__ . '/vendor/autoload.php';