Initial support for Laravel 8

[igibek]

Issue #33
I made changes according to official laravel documentation: https://laravel.com/docs/8.x/upgrade#seeder-factory-namespaces

[igibek]

This pull request introduces a breaking change for applications that uses Laravel 7. So, we need to create a new version of the package (3.0) and push it.

[MrCrayon]

@igibek I think we can detect Laravel version and use the appropriate folder, at the same time have a placeholder for {{namespace}} and replace it with the correct string.
This way we keep compatibility.
You can have a look at getSeedsFolderName in my PR for Voyager thedevdojo/voyager#5093