Remove support for passing assets as names

[morozov]

Q	A
Type	improvement
BC Break	yes

Summary

The current Platform and Schema APIs allow passing object names as strings or as objects themselves (sub-classes of Asset). For instance, in order to drop a table, the caller can pass the table name or the Table object.

While it may look handly from the API consumer perspective, it violates the Interface Segregation Principle. In the example above, the getDropTableSQL() method has to support accepting an object although it only needs the name of the object.

Another problem is that this API is prone to being implemented inconsistently depending on the type of the actually passed argument. For instance:

1. MySQL will drop the PK using ALTER TABLE tbl DROP PRIMARY KEY only if the index is passed as an object since the information about whether the given index is the PK or not isn't encoded in the index name:

   dbal/src/Platforms/MySQLPlatform.php

   Lines 897 to 901 in 27c26d2

   	if ($index instanceof Index && $index->isPrimary()) {
   	// MySQL primary keys are always named "PRIMARY",
   	// so we cannot use them in statements because of them being keyword.
   	return $this->getDropPrimaryKeySQL($table);
   	}

2. The Table class will validate the columns of the foreign key against the referenced table only if the referenced table name is passed as an asset:

   dbal/src/Schema/Table.php

   Lines 354 to 360 in f83121f

   	if ($foreignTable instanceof Table) {
   	foreach ($foreignColumnNames as $columnName) {
   	if (! $foreignTable->hasColumn($columnName)) {
   	throw ColumnDoesNotExist::new($columnName, $foreignTable->getName());
   	}
   	}
   	}

Another practical problem is that it cripples the Events API:

dbal/src/Event/SchemaDropTableEventArgs.php

Lines 34 to 37 in dc4ad2a

	/**
	* @return string|Table
	*/
	public function getTable()

The consumer of the SchemaDropTableEventArgs class has to support receiving the table name expressed as either of the string and Table.

As part of prototyping #4772, I'd like to convert all string $name arguments into Name $name. This is where the need to drop the support for Asset|string arguments arose.

TODO:

• Deprecate the previous API (Deprecate passing assets as names in 3.x #4798).
  The deprecation should happen after the Name-based API is implemented or before 4.0.x is released (whatever happens first).

[mvorisek]

Very good!