Vim recently has introduced a new feature called 'packages', which
permits, among the rest, to disable automatic loading a plugin at
startup. Plugins that are disabled at startup may be manually loaded
during a session using :packadd (see :h packages, :h :packadd). Of
course, a plugin that is loaded after startup cannot rely on VimEnter
events.
In order to support lazy loading YCM, check whether YCM is sourced
during startup or at a later time. In the former case, define an
autocommand as before; in the latter case, enable YCM immediately.
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