Fix package load init order bugs

* Move calls to `minpac#init()` into commands which use minpac
* Check if optional package directories exist before using `packadd`

vimrc

@@ -12,8 +12,7 @@ if has('syntax') && !exists('g:syntax_on')
 endif
 
 if exists('*minpac#init')
-  " When minpac is loaded initialize and define the Pack command.
-  call minpac#init()
+  " When minpac is loaded define the Pack command to add packages.
   command! -nargs=+ Pack call minpac#add(<args>)
 else
   " Otherwise define the Pack command to do nothing.
@@ -203,11 +202,11 @@ Pack 'jrozner/vim-antlr'
 Pack 'greymd/oscyank.vim', {'type': 'opt'}
 let g:use_osc52 = $TMUX !=# '' &&
 \ trim(system('tmux show-options -g set-clipboard')) ==# 'set-clipboard on'
-if g:use_osc52
+if g:use_osc52 && isdirectory('pack/minpac/opt/oscyank.vim')
   packadd oscyank.vim
 endif
 
 " Load YouComplteMe if termdebug plugin is not loaded.
-if !exists(':Termdebug')
+if !exists(':Termdebug') && isdirectory('pack/minpac/opt/YouCompleteMe')
   packadd YouCompleteMe
 endif