fix(cmdline): cmdline completion of _defer_require() modules #33007

Problem:
`:lua vim.lsp.c<tab>` does not list vim.lsp.completion in the completion
list after 24cea4c7f7417c7fe99a98a0487f51dd68c4f409.

Solution:
- Always include `vim.lsp._submodule` keys in candidates.
  - Fixes `vim.lsp.c<tab>` -> `vim.lsp.completion`.
- Eager-load `vim.lsp.completion` to get its completion.
  - Fixes `vim.lsp.completion.g<tab>` -> `vim.lsp.completion.get`.

1 files changed, 7 insertions, 0 deletions

diff --git a/test/functional/editor/completion_spec.lua b/test/functional/editor/completion_spec.lua
index 106e0df347..3e5d0e48e8 100644
--- a/test/functional/editor/completion_spec.lua
+++ b/test/functional/editor/completion_spec.lua
@@ -928,6 +928,13 @@ describe('completion', function()
       command('set wildoptions+=fuzzy')
       eq({ 'vim' }, fn.getcompletion('vi', 'lua'))
     end)
+
+    it('completes _defer_require() modules', function()
+      -- vim.lsp.c<tab> -> vim.lsp.completion
+      ok(vim.tbl_contains(fn.getcompletion('lua vim.lsp.c', 'cmdline'), 'completion'))
+      -- vim.lsp.completion.g<tab> -> vim.lsp.completion.get
+      ok(vim.tbl_contains(fn.getcompletion('lua vim.lsp.completion.g', 'cmdline'), 'get'))
+    end)
   end)
 
   it('cmdline completion supports various string options', function()