DirectoryListing: fix generic declarations

[JanTvrdik]

• marks T as covariant, this is required to make DirectoryListing<FileAttributes> subtype of DirectoryListing<StorageAttributes>
• add missing upper bound for T, because sortByPath relies on items being subtypes of StorageAttributes
• add missing generic parameter for implemented interface IteratorAggregate

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playground

[SamMousa]

I don't think DirectoryListing should even be a generic.
DirectoryListing is a collection with objects of type StorageAttributes. Each instance of this collection will very likely contain multiple implemenations, specifically FileAttributes and DirectoryAttributes. After all a directory can contain both directories and files.

So in conclusion I believe both the current annotation and this PR are wrong. What do you think?

[JanTvrdik]

That would also be an option, but there are some usecase for keeping it generic, i.e. you could do sth like

    /**
     * @param  DirectoryListing<StorageAttributes> $listing
     * @return DirectoryListing<FileAttributes>
     */
    private function getFilesInDirectory(DirectoryListing $listing): DirectoryListing
    {
        return $listing->filter(static function (StorageAttributes $attributes): bool {
            return $attributes->isFile();
        });
    }