Call UrlHelper::isExternal before Url::fromUri('internal:' . $path)

Instead of duplicating the call to isExternal in both places just to return an exception, I opted for the already existing exception from Url::fromUri to bubble up and only moved the logic check into processOutbound I tend to shy away from duplicate calls, especially when it entails mildly expensive regex calls on every single page.

The last commit reverts back to the previous approach. cl_server has an AbsolutePathProcessor that executes for URL generation. Which proves that we can have internal URLs that are absolute by the time they get to GroupPathPrefixRepository::getGroupByInternalPath. Let's guard against that.

Automatically closed - issue fixed for 2 weeks with no activity.