rplugin: Call s:LoadRemotePlugins() on startup.

Dispense with the FuncUndefined/CmdUndefined quasi-optimization.
If there are no rplugins, plugin/rplugin.vim takes less than 1ms.

Closes #5821
Closes #6250

Helped-by: Qiming zhao <chemzqm@gmail.com>

runtime/plugin/rplugin.vim

@@ -1,16 +1,59 @@
 if exists('g:loaded_remote_plugins')
   finish
 endif
-let g:loaded_remote_plugins = 1
+let g:loaded_remote_plugins = '/path/to/manifest'
+
+" Get the path to the rplugin manifest file.
+function! s:GetManifestPath() abort
+  let manifest_base = ''
+
+  if exists('$NVIM_RPLUGIN_MANIFEST')
+    return fnamemodify($NVIM_RPLUGIN_MANIFEST, ':p')
+  endif
+
+  let dest = has('win32') ? '$LOCALAPPDATA' : '$XDG_DATA_HOME'
+  if !exists(dest)
+    let dest = has('win32') ? '~/AppData/Local' : '~/.local/share'
+  endif
+
+  let dest = fnamemodify(expand(dest), ':p')
+  if !empty(dest) && !filereadable(dest)
+    let dest .= ('/' ==# dest[-1:] ? '' : '/') . 'nvim'
+    call mkdir(dest, 'p', 0700)
+    let manifest_base = dest
+  endif
+
+  return manifest_base.'/rplugin.vim'
+endfunction
+
+" Old manifest file based on known script locations.
+function! s:GetOldManifestPath() abort
+  let prefix = exists('$MYVIMRC')
+        \ ? $MYVIMRC
+        \ : matchstr(get(split(execute('scriptnames'), '\n'), 0, ''), '\f\+$')
+  return fnamemodify(expand(prefix, 1), ':h')
+        \.'/.'.fnamemodify(prefix, ':t').'-rplugin~'
+endfunction
+
+function! s:GetManifest() abort
+  let manifest = s:GetManifestPath()
+  if !filereadable(manifest)
+    " Check if an old manifest file exists and move it to the new location.
+    let old_manifest = s:GetOldManifestPath()
+    if filereadable(old_manifest)
+      call rename(old_manifest, manifest)
+    endif
+  endif
+  return manifest
+endfunction
+
+function! s:LoadRemotePlugins() abort
+  let g:loaded_remote_plugins = s:GetManifest()
+  if filereadable(g:loaded_remote_plugins)
+    execute 'source' fnameescape(g:loaded_remote_plugins)
+  endif
+endfunction
 
 command! UpdateRemotePlugins call remote#host#UpdateRemotePlugins()
 
-augroup nvim-rplugin
-  autocmd!
-  autocmd FuncUndefined *
-        \ call remote#host#LoadRemotePluginsEvent(
-        \   'FuncUndefined', expand('<amatch>'))
-  autocmd CmdUndefined *
-        \ call remote#host#LoadRemotePluginsEvent(
-        \   'CmdUndefined', expand('<amatch>'))
-augroup END
+call s:LoadRemotePlugins()