Author: Paul Holden
MDL-86915 core: define "core" plugin type string.
Per ae7f3dfd this is now required for all plugin types. In this
specific case, it's used by the privacy plugin registry.
http://git.moodle.org/gw?p=moodle.git;a=commit;h=e69e34174cea0fbf726eb2b797cbd5897ca1b59d
http://github.com/moodle/moodle/commit/e69e34174cea0fbf726eb2b797cbd5897ca1b59d
+ 5.1 en [type_core,core_plugin]
+ 5.1 en [type_core_plural,core_plugin]