Some are not as specific as they should be, and some may even be wrong,
but since these are only type hints (not type declarations) they won't
affect code execution, but rather provide some help to better understand
the code – for developers as well as tooling.
`Fetcher::getByUri()` returns a string (and since it is a public method
we don't want to change that for now), but we check for `false`; so we
change that.
We also use `file_exists()` instead of `Cache::isFileCached()` because
the method is actually called `Cache::fileIsCached()` but appears to be
broken due to applying `::getCacheablePath()` again. Furthermore the
other methods of `Cache` also use `file_exists()` directly. Since
`Cache::fileIsCached()` is public, we do not remove the otherwise
unused method yet.
