[entropy.const] introduce const_file_readable()
[entropy.core] use const_file_readable()
[entropy.fetchers] use const_file_readable(), drop access() usage
[entropy.const] const_file_readable: fix typo in docstring
[entropy.server.interfaces.db] use const_file_readable(), drop access() usage
[entropy.const] introduce const_dir_readable()
[entropy.server.interfaces.mirrors] drop access() usage
[entropy.spm] drop access() usage