Move from modules support to subsystems and all plugin types support

Not only modules are interested in indexing their contents in a search engine, we should allow all moodle components (subsystems and all plugin types) to define what contents they want to index.

It is very important to document how components can add support for global search as each component will be interested in indexing different kind of components.

• module and module link fields in search/solr/solr_conf/N.x/schema.xml can be changed for component and componentlink in this issue

• search_get_iterators() and all functions gathering the current mod/PLUGIN/lib.php functions (https://github.com/dmonllao/moodle/compare/MDL-31989_master#diff-378b58ec79512fa90282e9a6fb7a5983R80) should be changed to read, for example, a mod/PLUGIN/db/search.php file

It is out of the scope of this issue to add global search support to other components than modules and course, we will have time for this later, but it is important to define a format that can be used accross components.