Fix directory detection through symlinks

[Voltra]

Initially this PR (#125) was meant to correct an issue on Windows where symlink'd directories would be considered neither as a directory, nor a symlink.

It has since grown to ensure that symlink'd directories are properly exported on both Windows and UNIX platforms. This is important as php artisan storage:link creates such a symlink and users will very likely want to export it as if it was a regular directory.

On Windows, stepping into a debugger revealed that SplFileInfo::isDir(), SplFileInfo::isFile() and SplFileInfo::isLink() all returned false on that specific entry. On UNIX platforms, SplFileInfo::isDir() and SplFileInfo::isLink() both return true where SplFileInfo::isFile() returns false.

However, SplFileInfo::getRealPath() does yield the correct directory path (i.e. the one the symlink points to).

As such, this PR simply makes directory detection portable on all OS that could suffer from this specific issue without hindering the process for other types of files.

[Voltra]

NB: I haven't taken the time to run the Windows-specific test on a Windows machine, but I've been using my PR as a local patch on a personal project without any issue this far

[freekmurze]

I don't want to maintain this complexity in our package, so I'm going to pass on this for now and add a note in the readme that it will only work Unix systems.

[Voltra]

I don't want to maintain this complexity in our package, so I'm going to pass on this for now and add a note in the readme that it will only work Unix systems.

With all due respect, as stated in the body of the PR, the package in its current state still fails to properly export UNIX symlinked directories

[Voltra]

Something you can assert by running the test suite from the PR on an unpatched version of the package

[Voltra]

I also feel a bit disrespected for being asked to write tests (on #125) to then have the fix dismissed here instead of dismissing it right away