Problem:
`:lua vim.lsp.c<tab>` does not list vim.lsp.completion in the completion
list after 24cea4c7f7.
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`.
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