Symfony4 Error loading classes custom folder "Expected to find class... but it was not found" : <h2>Problem</h2>

<p>I'm trying to setup a custom directory structure 
for some shared classes in my Symfony project. I
want to create a custom folder in the root of my
project and I want to use the Symfony auto-load 
feature to automatically register services from 
that folder.</p>

<p>So I added a custom services namespace to the
services.yaml file:</p>

<pre><code># src ./config/services.yaml
services:
    ...

    TestNamespace\:
    resource: '../TestNamespace/*'

  ...
</code></pre>

<p>And I added an empty class in the custom folder:</p>

<pre><code># src ./TestNamespace/TestClass.php

namespace TestNamespace;

class TestClass
{

}
</code></pre>

<p>When I run the app I get the following error:</p>

<pre><code>(1/2) InvalidArgumentException
Expected to find class "TestNamespace\TestClass" in file 
"/path/to/ClassLoadErrorDemo/demo/TestNamespace/TestClass.php"
while importing services from resource 
"../TestNamespace/*", but it was not found! Check the
namespace prefix used with the resource.

(2/2) FileLoaderLoadException
Expected to find class "TestNamespace\TestClass" in file 
"/path/to/ClassLoadErrorDemo/demo/TestNamespace/TestClass.php" while 
importing services from resource "../TestNamespace/*", but it was not 
found! Check the namespace prefix used with the resource in 
/path/to/ClassLoadErrorDemo/demo/config/services.yaml (which is loaded 
in resource "/path/to/ClassLoadErrorDemo/demo/config/services.yaml").
</code></pre>

<p>I double checked the paths, namespace and the class 
name multiple times and everything seems fine and I 
don't understand why I still get the error.
Controllers in the ./src folder seem to load fine.
What am I doing wrong here? </p>

<h2>Steps to reproduce</h2>

<p>I created a demo repo to isolate the problem.</p>

<pre><code>git clone https://github.com/smoelker/SymfonyClassLoadErrorDemo.git
cd SymfonyClassLoadErrorDemo/demo
composer install
mv TestNamespace/TestClass.php_ TestNamespace/TestClass.php
php bin/console server:start
</code></pre>
