Symfony 4 without top level namespace - symfony

Question

I want to migrate a Silex project to Symfony 4. I'm working on the skeleton first so I executed:
composer create-project symfony/skeleton myProject
and then required the bundles with composer require bundle for each one.
One of them needs some configuration to be installed, so I added a file config/packages/custom_bundle.yaml with such configuration.
custom_bundle.yaml needs a custom user provider service which I copied from the Silex project to the same namespace (Security) with the same name (UserProvider.php) so I created a service at config/services.yaml:
security.users_provider:
class: Security\UserProvider
Then I tried to require the bundle again and I get this error:
The autoloader expected class "App\Security\UserProvider" to be defined in file "/vagrant/myProject/vendor/composer/../../src/Security/UserProvider.php".
The file was found but the class was not in it, the class name or namespace probably has a typo in /vagrant/myProject/config/services.yaml (which is loaded in resource "/vagrant/myProject/config/services.yaml").
Of course, I have to tell composer to load classes from non-top-level-namespace so I replaced:
"autoload": {
"psr-4": {
"App\\": "src/"
}
},
in composer.json with:
"autoload": {
"psr-4": {
"": "src/"
}
},
But now I get this error because the Kernel.php namespace is App:
PHP Fatal error: Uncaught Symfony\Component\Debug\Exception\ClassNotFoundException: Attempted to load class "Kernel" from namespace "App".
I have the entire Silex project without top-level-namespace, so adding App or whatever as top-level-namespace would mean renaming the whole project classes.
By other hand, removing the App namespace of Kernel.php would mean using this class as global, which might won't be a good idea.
Any workaround to avoid rewriting all classes namespaces?
PS: I also tried with:
"autoload": {
"psr-4": {
"App\\": "src/",
"": "src/",
}
},
but same happens.
Edit:
The problem was the default definition of services:
App\:
resource: '../src/*'
exclude: '../src/{Entity,Migrations,Tests,Kernel.php}'
As I don't have the App namespace I've had to comment it by now. Later I'll have to add any services to be auto-defined with their namespaces. E.g.
Service\:
resource: '../src/Service/*'
So it works with commenting the default App services definition and adding:
"autoload": {
"psr-4": {
"App\\": "src/",
"Secutiry\\": "src/Secutiry/",
}
},
and any other namespaces to composer.json. Thanks #Cerad for the tips.
Edit2:
Even works with:
"autoload": {
"psr-4": {
"App\\": "src/",
"": "src/"
}
},