[8.x] Allow to override discover events base path

[rodrigopedra]

When customizing the app's directory structure event discovery fails as it uses base_path() to remove the "real path" part from a class file and tries to guess the class name from the remaining path.

For example, if one is following "Laravel Beyond Crud" book, from Spatie, proposed file structure, and have an event placed at ./src/App/Events/UserCreated.php, the DiscoverEvents class will guess the namespace of that class as Src\App\Events\UserCreated, which PHP won't find as a class.

This PR allows a developer to override in their local EventServiceProvider the base path used for event discovery by adding a protected method they can override and provide a custom base path to be used by the DiscoverEvents class.

EDIT no tests were added as the test for DiscoverEvents is already agnostic about the base path used.

reference:

framework/tests/Integration/Foundation/DiscoverEventsTest.php

Lines 15 to 32 in 0798479

	public function testEventsCanBeDiscovered()
	{
	class_alias(Listener::class, 'Tests\Integration\Foundation\Fixtures\EventDiscovery\Listeners\Listener');
	class_alias(AbstractListener::class, 'Tests\Integration\Foundation\Fixtures\EventDiscovery\Listeners\AbstractListener');
	class_alias(ListenerInterface::class, 'Tests\Integration\Foundation\Fixtures\EventDiscovery\Listeners\ListenerInterface');
	$events = DiscoverEvents::within(__DIR__.'/Fixtures/EventDiscovery/Listeners', getcwd());
	$this->assertEquals([
	EventOne::class => [
	Listener::class.'@handle',
	Listener::class.'@handleEventOne',
	],
	EventTwo::class => [
	Listener::class.'@handleEventTwo',
	],
	], $events);
	}