DX: Prevent having deprecated fixers listed as successors of other deprecated fixers

[Wirone]

Revived #7960, credits to @kubawerlos.

In terms of this discussion, I believe the point was valid but maybe not articulated correctly. As you can see, AbstractFixerTestCase::testDeprecatedFixersDoNotHaveDeprecatedSuccessor() iterates over the result of DeprecatedFixerInterface::getSuccessorsNames() and then calls TestCaseUtils::getFixerByName() (previously this method was a part of TestCase). The problem here is that getSuccessorsNames()'s return type is defined as list<string> and there is not any mechanism that would verify if successors are valid references to existing fixers. For example, when we do:

public function getSuccessorsNames(): array
{
    return array_merge(array_keys($this->proxyFixers), ['foo']);
}

the docs are generated properly, Auto Review tests pass, everything looks OK, but then testDeprecatedFixersDoNotHaveDeprecatedSuccessor() fails with TypeError: PhpCsFixer\Tests\Test\TestCaseUtils::getFixerByName(): Return value must be of type PhpCsFixer\Fixer\FixerInterface, null returned.

With the explicit check, as suggested in the original PR, it just looks better and is more descriptive:

PS. Maybe we should have auto-review test that would verify if defined successors are valid fixers? (should be done in separate PR, if needed)

[coveralls]

coverage: 96.113% (+0.08%) from 96.03%
when pulling 18f160a on Wirone:codito/revive-7960
into 37d8d45 on PHP-CS-Fixer:master.

[julienfalque]

isset is more performant I think:

Suggested change

	if (!\array_key_exists($name, $fixers)) {
	if (!isset($fixers[$name])) {

[Wirone]

Oops, "nice to have" suggestions don't work nicely with auto-merge 😅.

[keradus]

suggestion - ignore this single usage, confirm what is more performant, apply locally with CI check