[8.x] Add nested relationships to whereRelation function

[josemariagomez]

This PR added the possibility to use nested relationships when using 'whereRelation' Eloquent's function, using dot notation.
Also still works with not nested relationships, so it's not a breaking change.

How it works

// Before
Company::whereHas('teams', function($query) {
  $query->whereHas('users', function($query) {
      $query->whereHas('roles', function($query) {
          $query->where('name', 'admin');
      });
  });
})->get();

// After 💅🏻
Company::whereRelation('teams.users.roles', 'name', 'admin')->get();

// Also can be used as orWhereRelation
Company::where('public', true)->orWhereRelation('teams.users', 'id', auth()->id())->get();

// Still can use this function for not nested relationships
Company::whereRelation('teams', 'id', '1')->get();

[devfrey]

Why was this merged?

1. It was already possible to used nested relations before these code changes.
2. This PR contains no tests...