Problem: Plain `vim.pack.add()` calls (with default `opts.load`) does
not fully work if called inside 'plugin/' runtime directory. In
particular, 'plugin/' files of newly added plugins are not sourced.
This is because `opts.load` is `false` during the whole startup, which
means `:packadd!` is used (modify 'runtimepath' but not force source
newly added 'plugin/' files).
This use case is common due to users organizing their config as
separate files in '~/.config/nvim/plugin/'.
Solution: Use newly added `v:vim_did_init` to decide default `opts.load`
value instead of `v:vim_did_enter`.
a39171f532