feat(lua): omnifunc for builting lua interpreter

also make implicit submodules "uri" and "_inspector" work with completion

this is needed for `:lua=vim.uri_<tab>` wildmenu completion
to work even before uri or _inspector functions are used.

runtime/lua/vim/_editor.lua

@@ -42,6 +42,18 @@ for k, v in pairs({
   vim._submodules[k] = v
 end
 
+-- There are things which have special rules in vim._init_packages
+-- for legacy reasons (uri) or for performance (_inspector).
+-- most new things should go into a submodule namespace ( vim.foobar.do_thing() )
+vim._extra = {
+  uri_from_fname = true,
+  uri_from_bufnr = true,
+  uri_to_fname = true,
+  uri_to_bufnr = true,
+  show_pos = true,
+  inspect_pos = true,
+}
+
 vim.log = {
   levels = {
     TRACE = 0,
@@ -575,15 +587,13 @@ function vim._on_key(char)
 end
 
 --- Generate a list of possible completions for the string.
---- String starts with ^ and then has the pattern.
+--- String has the pattern.
 ---
 ---     1. Can we get it to just return things in the global namespace with that name prefix
 ---     2. Can we get it to return things from global namespace even with `print(` in front.
 function vim._expand_pat(pat, env)
   env = env or _G
 
-  pat = string.sub(pat, 2, #pat)
-
   if pat == '' then
     local result = vim.tbl_keys(env)
     table.sort(result)
@@ -644,7 +654,7 @@ function vim._expand_pat(pat, env)
       local mt = getmetatable(final_env)
       if mt and type(mt.__index) == 'table' then
         field = rawget(mt.__index, key)
-      elseif final_env == vim and vim._submodules[key] then
+      elseif final_env == vim and (vim._submodules[key] or vim._extra[key]) then
         field = vim[key]
       end
     end
@@ -674,6 +684,7 @@ function vim._expand_pat(pat, env)
   end
   if final_env == vim then
     insert_keys(vim._submodules)
+    insert_keys(vim._extra)
   end
 
   keys = vim.tbl_keys(keys)
@@ -745,6 +756,28 @@ vim._expand_pat_get_parts = function(lua_string)
   return parts, search_index
 end
 
+do
+  -- Ideally we should just call complete() inside omnifunc, though there are
+  -- some bugs, so fake the two-step dance for now.
+  local matches
+
+  --- Omnifunc for completing lua values from from the runtime lua interpreter,
+  --- similar to the builtin completion for the `:lua` command.
+  ---
+  --- Activate using `set omnifunc=v:lua.vim.lua_omnifunc` in a lua buffer.
+  function vim.lua_omnifunc(find_start, _)
+    if find_start == 1 then
+      local line = vim.api.nvim_get_current_line()
+      local prefix = string.sub(line, 1, vim.api.nvim_win_get_cursor(0)[2])
+      local pos
+      matches, pos = vim._expand_pat(prefix)
+      return (#matches > 0 and pos) or -1
+    else
+      return matches
+    end
+  end
+end
+
 ---Prints given arguments in human-readable format.
 ---Example:
 ---<pre>lua