MDL-38391 lib: Rename moodle_metadata adder

This is a bit of a grey area. This function does not get and return a value
to the caller, and does not set a value taken from the caller. Instead it
gets a value from get_moodle_metadata and adds it to the current instance
of YUI_config. This rename will hopefully improve clarity here.
MDL-38391 lib: Also include core subsystems in YUI_config.modules