Make service provider deferrable to allow for custom (filesystem) service provider

[PrinsFrank]

As described on a similar PR in the ide-helper, when a serviceprovider is not deferrable, the service provider of a package is registered before any custom service providers in a project are registered.

I am currently working on a project with a custom FileSystemServiceProvider, where requiring the debugbar causes an error in Container.php on line 879 Target class [files] does not exist and Container.php on line 877 Class "files" does not exist. This issue was also mentioned in #1112.

Simplest bug reproduction in code

./app/Providers/CustomFileSystemProvider.php

<?php
declare(strict_types=1);

namespace App\Providers;

use Illuminate\Filesystem\FilesystemServiceProvider;

class CustomFileSystemServiceProvider extends FilesystemServiceProvider
{
    // Overwrite anything in here, or keep this empty for a simple reproduction
}

./config/app.php

'providers' => [
    //Illuminate\Filesystem\FilesystemServiceProvider::class,
    App\Providers\CustomFileSystemServiceProvider::class,

This PR adds the DeferrableProvider interface to the package's service provider, and adds the provided services to a provides method so these services can be registered when needed.

[caimaxtingle]

+1 also getting this issue, please approve asap

[PrinsFrank]

@barryvdh Can you look at this PR?

[PrinsFrank]

@barryvdh Can this get merged? Our team has been running on this fork since november with no issues, and there are is also someone else running into this issue in #1306.

[PrinsFrank]

As #1197 is now merged I'll close this PR.