refactor: create function for deferred loading

The benefit of this is that users only pay for what they use. If e.g.
only `vim.lsp.buf_get_clients()` is called then they don't need to load
all modules under `vim.lsp` which could lead to significant startuptime
saving.

Also `vim.lsp.module` is a bit nicer to user compared to
`require("vim.lsp.module")`.

This isn't used for some nested modules such as `filetype` as it breaks
tests with error messages such as "attempt to index field 'detect'".
It's not entirely certain the reason for this, but it is likely it is
due to filetype being precompiled which would imply deferred loading
isn't needed for performance reasons.

runtime/lua/vim/lsp/diagnostic.lua

@@ -1,8 +1,6 @@
 ---@brief lsp-diagnostic
 
-local util = require('vim.lsp.util')
 local protocol = require('vim.lsp.protocol')
-local log = require('vim.lsp.log')
 local ms = protocol.Methods
 
 local api = vim.api
@@ -95,7 +93,7 @@ local function tags_lsp_to_vim(diagnostic, client_id)
       tags = tags or {}
       tags.deprecated = true
     else
-      log.info(string.format('Unknown DiagnosticTag %d from LSP client %d', tag, client_id))
+      vim.lsp.log.info(string.format('Unknown DiagnosticTag %d from LSP client %d', tag, client_id))
     end
   end
   return tags
@@ -425,7 +423,7 @@ end
 local function _refresh(bufnr, opts)
   opts = opts or {}
   opts['bufnr'] = bufnr
-  util._refresh(ms.textDocument_diagnostic, opts)
+  vim.lsp.util._refresh(ms.textDocument_diagnostic, opts)
 end
 
 --- Enable pull diagnostics for a buffer