[8.x] Modify has() method to support MorphTo relations

[khalilst]

This PR modifies the has() method with min changes to support MorphTo relations.
Also, the following descendants of has() method inherit this feature:

protected function hasNested(...)
public function orHas(...)
public function doesntHave(...)
public function orDoesntHave(...)
public function whereHas(...)
public function orWhereHas(...)
public function whereDoesntHave(...)

In addition to the above feature the hasMorph() method and its descendants support the MorphTo relation as first argument (MorphTo|string):

/**
 * Add a polymorphic relationship count / exists condition to the query.
 *
 * @param  \Illuminate\Database\Eloquent\Relations\MorphTo|string  $relation
 * ....
 */
public function hasMorph($relation, ...)

public function orHasMorph($relation,...)
public function doesntHaveMorph($relation, ...)
public function whereHasMorph($relation, ...)
public function orWhereHasMorph($relation, ...)
public function orHasMorph($relation, ...)
public function doesntHaveMorph($relation, ...)
public function orDoesntHaveMorph($relation,...)