[Configuration] Resolve --only rule name with shell-eaten backslashes

[TomasVotruba]

Running this in a shell fails, because zsh/bash strip the unescaped backslashes before Rector ever sees the argument:

bin/rector p --only \Rector\CodeQuality\Rector\FuncCall\ChangeArrayPushToArrayAssignRector

In OnlyRuleResolver.php line 61:

  Rule "RectorCodeQualityRectorFuncCallChangeArrayPushToArrayAssignRector" 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"

The rule name is correct, only the separators are gone. This adds a last-resort fallback: when the passed name contains no backslash at all, compare it against registered rules with their backslashes stripped. If exactly one matches, use it.

-$ bin/rector p --only \Rector\CodeQuality\Rector\FuncCall\ChangeArrayPushToArrayAssignRector
-  Rule "RectorCodeQualityRectorFuncCallChangeArrayPushToArrayAssignRector" was not found.
+$ bin/rector p --only \Rector\CodeQuality\Rector\FuncCall\ChangeArrayPushToArrayAssignRector
+  [OK] Rector is done!

Ambiguous or unknown flattened names still throw the same errors as before.