[Configuration] Report explicit error when --only rule exists but is not registered

[TomasVotruba]

--only SomeRule reported "was not found" even when the rule class exists in the autoloaded code and is only missing from rector.php. Now the rule class is looked up (FQCN via ReflectionProvider, short name via a scan of Rector PSR-4 dirs) and the error says so explicitly.

Before:

Rule "SafeDeclareStrictTypesRector" was not found.
The rule has no namespace. Make sure to escape the backslashes, and add quotes around the rule name: --only="My\Rector\Rule"

After:

Rule "Rector\TypeDeclaration\Rector\StmtsAwareInterface\SafeDeclareStrictTypesRector" exists, but is not registered in your Rector config.
Register it in your rector.php:

    ->withRules([SafeDeclareStrictTypesRector::class])

Unknown rule names keep the original messages.