Before this patch, whenever core_plugin_manager::get_plugins() was
called, it always attached info about available updates. But this is
needed only in quite rare cases, such as when the admin is looking at
the Plugins overview and Plugins check screens. There is no need to load
this on other places and for non-admin users.

The patch removes the loading from the method
core_plugin_manager::get_plugins_of_type() and implements lazy loading
directly in the plugininfo classes so that it is loaded only when
\core\plugininfo\base::available_updates() is actually called.