cascache.py: use move_atomic instead of manual error checking

            checkoutdir = os.path.join(tmpdir, ref)

            self._checkout(checkoutdir, tree)

            os.makedirs(os.path.dirname(dest), exist_ok=True)

            try:

                os.rename(checkoutdir, dest)

                utils.move_atomic(checkoutdir, dest)

            except DirectoryExistsError:

                # Another process beat us to renamse

                pass

            except OSError as e:

                # With rename it's possible to get either ENOTEMPTY or EEXIST

                # in the case that the destination path is a not empty directory.

                #

                # If rename fails with these errors, another process beat

                # us to it so just ignore.

                if e.errno not in [errno.ENOTEMPTY, errno.EEXIST]:

                raise CASError("Failed to extract directory for ref '{}': {}".format(ref, e)) from e

        return dest