[5.5] Updated Console/Kernel autoloading of commands to ignore abstract classes

[davidrushton]

The new Laravel 5.5 auto loading of console commands is great.

Unfortunately it didn't work in my app, as I use an abstract class that extends Illuminate\Console\Command, so the is_subclass_of($command, Command::class) was resolving that abstract class causing a Illuminate\Contracts\Container\BindingResolutionException exception.

This PR changes the conditional to use ReflectionClass and the isAbstract and isSubclassOf methods, fixing this issue.

PS. My abstract console class is extended by custom migration classes that are necessary for our multi-tenant / multi-database migrations, while the base migrator commands act on our primary database.