Do not report a private method overriding a private trait method as unused

[Jean-Beru]

Fixes phpstan/phpstan#12201

Why

A class member takes precedence over the member of the same name coming from a used trait. So a private method redeclared in the class is the one the trait's own methods call.

UnusedPrivateMethodRule sees those call sites only when the trait is part of the analysed files. In a project whose paths exclude its dependencies, ClassMethodsNode::getMethodCalls() collects no call at all. The class method is then reported as unused.

The canonical case is a Symfony application. The recipe generates a Kernel that redeclares KernelTrait::getAllowedEnvs(), and the trait lives in vendor/.

// vendor/symfony/dependency-injection/Kernel/KernelTrait.php, not analysed
trait KernelTrait
{
	private function getAllowedEnvs(): array
	{
		return [];
	}

	protected function getKernelParameters(): array
	{
		// ...
		if (!$knownEnvs = array_flip($this->getAllowedEnvs())) {
		// ...
	}
}

// src/Kernel.php, analysed
class Kernel extends BaseKernel
{
	use MicroKernelTrait; // uses KernelTrait

	/**
	 * @return list<string>
	 */
	private function getAllowedEnvs(): array // reported as unused
	{
		return ['prod', 'dev', 'test'];
	}
}

Every new Symfony 8.1 project hits this from level 4 up. ignoreErrors is the only way out.

What

A private method is skipped when a used trait declares a private method of the same name.

getTraits() is enough without recursion. PHP flattens trait composition. getAllowedEnvs() comes from the nested KernelTrait, and the reflection of MicroKernelTrait already reports it.