Support lazy loading with :packadd.
- [X] I have read and understood YCM's [CONTRIBUTING][cont] document.
- [X] I have read and understood YCM's [CODE_OF_CONDUCT][code] document.
- [X] I have included tests for the changes in my PR. If not, I have included a
rationale for why I haven't.
- [X] **I understand my PR may be closed if it becomes obvious I didn't
actually perform all of these steps.**
# Why this change is necessary and useful
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.
Why would one want to load YCM lazily? I use YCM only for certain file
types (Python), and I prefer not to have pop up menus appearing
all the time for other file types. In my workflow, I may enable YCM only
if I start Vim for Python coding sessions.
Tests not included because the change is in VimScript code only, and
testing this feature is not trivial.
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