Consistently use either is_file() or file_exists()

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Thanks!

I think this is probably still valid, but we only really care about it in the critical path and it's not straightforward when checking for things that might be directories, so we can't enforce it.

So I think we should change DrupalKernal::findSitePath() to use is_file(), since that runs on every single request to Drupal. And not worry too much about anywhere else. Asset aggregation used to use file_exists() on every request 12 years ago but don't any more.