Case-sensitive path existence check

        >>> sdist_add_defaults._cs_path_exists(__file__)
        True
        >>> sdist_add_defaults._cs_path_exists(__file__.upper())
        False

def _cs_path_exists(fspath):
        """
        Case-sensitive path existence check

        >>> sdist_add_defaults._cs_path_exists(__file__)
        True
        >>> sdist_add_defaults._cs_path_exists(__file__.upper())
        False
        """
        if not os.path.exists(fspath):
            return False
        # make absolute so we always have a directory
        abspath = os.path.abspath(fspath)
        directory, filename = os.path.split(abspath)
        return filename in os.listdir(directory)