[5.8] Method on EloquentUserProvider to create query builder instance

[epalmans]

Reason: allows easy extension for a custom eloquent based auth provider in case, for example, you'd only need to have an extra constraint on the backing model applied. Prevents the need to fully implement the UserProvider interface.

Explanation of changes in EloquentUserProvider: instead of calling newQuery() on the model instance directly, now done through a protected method that can easily be overriden.

Example implementation where someone would have an "admin" guard that should only validate when a extra criteria (a field on the users table) is set:

<?php

namespace App\Extensions;

use Illuminate\Auth\EloquentUserProvider;

class MyEloquentAdminProvider extends EloquentUserProvider
{
    protected function modelQuery($model = null)
    {
        return parent::modelQuery($model)->where('role', 'admin');
    }
}

[epalmans]

note to maintainers: new commit 391eae8 adds back in a newQuery call (through the new modelQuery method) that was taken out at f74c83e