SourceLayout: merge helpers/ into src/

* Move helpers/basic_auth/ into src/auth/basic/

* Move helpers/digest_auth/ into src/auth/digest/

* Move helpers/external_acl/ into src/acl/external/

* Move helpers/log_daemon/ into src/log/

* Move helpers/negotiate_auth/ into src/auth/negotiate/

* Move helpers/ntlm_auth/ into src/auth/ntlm/

* Move helpers/storeid_rewrite/ into src/store/id_rewriters/

* Move helpers/url_rewrite/ into src/http/url_rewriters/

* Rename helpers/defines.h to src/helper/protocol_defines.h