Deprecate dropping columns referenced by constraints

[morozov]

Q	A
Type	deprecation

Summary

Currently, it is impossible to reference a non-existing column in a constraint being added but it's possible to drop the column afterwards, which will leave the table in an invalid state.

The order of operations in the following test will have to be changed once this deprecation is promoted to exception:

dbal/tests/Schema/TableTest.php

Lines 823 to 825 in 33f5589

	$table->dropColumn($assetName);
	$table->dropIndex($assetName);
	$table->removeForeignKey($assetName);

The following is out of scope of this PR:

1. Renaming the column being renamed in referencing indexes and constraints (Rename column in indexes and constraints #6578).
2. Dropping the column being dropped from referencing indexes and dropping the eventually empty indexes.

[derrabus]

To what extend do we want schema objects to perform self-validation? Looking at the test code you've referenced, I'd say having an invalid table object is acceptable temporarily because the object might not only represent a deployed or deployable table, but also one that is being changed.

[morozov]

To what extend do we want schema objects to perform self-validation?

Eventually, I want the object to have to be always valid (see "make invalid states unrepresentable"). Otherwise, these objects are unusable in an invalid state, and there's no guarantee that this state will become valid at a later point.

What I also have in mind (as a DX improvement) is to separate the APIs for building and using tables (as well as other schema objects) by introducing builders and making objects immutable. This way, an object will be always guaranteed to be in a valid state, while the builder can be in an arbitrary state and allow arbitrary order of operations (win-win). The validation will happen once at the object construction time.

For example:

$table = getExistingTable();
$table->edit() // creates a builder
  ->makeSomeChange() // if this state is invalid, this is fine
  ->makeSomeOtherChange()
  ->build(); // a new table is created and validated

[greg0ire]

Thanks @morozov !