Reduce amount of total loaders

... and completely replace vim._load_package

This speeds up statements like:

  pcall(require, 'does.not.exist')

Since 'require' doesn't need to search through as many loaders and hence
results in less calls to nvim_get_runtime_file().

Resolves #20

lua/impatient.lua

@@ -118,6 +118,24 @@ local function load_package_with_cache(name, loader)
       return chunk
     end
   end
+
+  -- Copied from neovim/src/nvim/lua/vim.lua
+  for _, trail in ipairs(vim._so_trails) do
+    local path = "lua"..trail:gsub('?', basename) -- so_trails contains a leading slash
+    local found = vim.api.nvim_get_runtime_file(path, false)
+    if #found > 0 then
+      -- Making function name in Lua 5.1 (see src/loadlib.c:mkfuncname) is
+      -- a) strip prefix up to and including the first dash, if any
+      -- b) replace all dots by underscores
+      -- c) prepend "luaopen_"
+      -- So "foo-bar.baz" should result in "luaopen_bar_baz"
+      local dash = name:find("-", 1, true)
+      local modname = dash and name:sub(dash + 1) or name
+      local f, err = package.loadlib(found[1], "luaopen_"..modname:gsub("%.", "_"))
+      return f or error(err)
+    end
+  end
+
   return nil
 end
 
@@ -210,6 +228,21 @@ function M.clear_cache()
   os.remove(M.path)
 end
 
+-- -- run a crude hash on vim._load_package to make sure it hasn't changed.
+-- local function verify_vim_loader()
+--   local expected_sig = 31172
+
+--   local dump = {string.byte(string.dump(vim._load_package), 1, -1)}
+--   local actual_sig = #dump
+--   for i = 1, #dump do
+--     actual_sig = actual_sig + dump[i]
+--   end
+
+--   if actual_sig ~= expected_sig then
+--     print(string.format('warning: vim._load_package has an unexpected value, impatient might not behave properly (%d)', actual_sig))
+--   end
+-- end
+
 local function setup()
   if uv.fs_stat(M.path) then
     log('Loading cache file %s', M.path)
@@ -230,17 +263,17 @@ local function setup()
   local insert = table.insert
   local package = package
 
+  -- verify_vim_loader()
+
   -- Fix the position of the preloader. This also makes loading modules like 'ffi'
   -- and 'bit' quicker
   if package.loaders[1] == vim._load_package then
-    -- Move vim._load_package to the second position
-    local vim_load = table.remove(package.loaders, 1)
-    insert(package.loaders, 2, vim_load)
+    -- Remove vim._load_package and replace with our version
+    table.remove(package.loaders, 1)
   end
 
   insert(package.loaders, 2, load_from_cache)
   insert(package.loaders, 3, load_package_with_cache_reduced_rtp)
-  insert(package.loaders, 4, load_package_with_cache)
 
   vim.cmd[[
     augroup impatient

test/impatient_spec.lua

@@ -3,7 +3,6 @@ local helpers = require('test.functional.helpers')()
 local clear    = helpers.clear
 local exec_lua = helpers.exec_lua
 local eq       = helpers.eq
-local ok       = helpers.ok
 local cmd      = helpers.command
 
 local gen_exp = function(use_cachepack)
@@ -124,11 +123,15 @@ local gen_exp = function(use_cachepack)
 end
 
 describe('impatient', function()
-  before_each(function()
+  local function reset()
     clear()
     cmd [[set runtimepath=$VIMRUNTIME,.,./test]]
     cmd [[let $XDG_CACHE_HOME='scratch/cache']]
     cmd [[set packpath=]]
+  end
+
+  before_each(function()
+    reset()
   end)
 
   it('load plugins without impatient', function()
@@ -157,7 +160,7 @@ describe('impatient', function()
       eq({ 'Loading cache file scratch/cache/nvim/luacache' },
         exec_lua("return _G.__luacache.log"))
 
-      ok(use_cachepack == exec_lua("return _G.package.loaded['impatient.cachepack'] ~= nil"))
+      assert(use_cachepack == exec_lua("return _G.package.loaded['impatient.cachepack'] ~= nil"))
     end)
   end
 
@@ -169,4 +172,33 @@ describe('impatient', function()
     tests(true)
   end)
 
+  it('shouldn\'t be slow when loading missing modules', function()
+    os.execute[[rm -rf scratch/cache]]
+
+    local total_vim_load_dur = 0
+    local total_imp_load_dur = 0
+
+    local function load_no_exist_mod()
+      return exec_lua[[
+        local a = vim.loop.hrtime()
+        pcall(require, 'does.not.exist')
+        return (vim.loop.hrtime() - a)/1e6
+      ]]
+    end
+
+    for _ = 1, 20 do
+      reset()
+      local vim_load_dur = load_no_exist_mod()
+      total_vim_load_dur = total_vim_load_dur + vim_load_dur
+      exec_lua[[require('impatient')]]
+      local imp_load_dur = load_no_exist_mod()
+      total_imp_load_dur = total_imp_load_dur + imp_load_dur
+    end
+
+    local threshold = 1.2 * total_vim_load_dur
+
+    assert(total_imp_load_dur < threshold,
+      string.format('%f > %f', total_imp_load_dur, threshold))
+  end)
+
 end)

test/lua/plugins.lua

@@ -26,9 +26,9 @@ for plugin, sha in pairs(init) do
     'rev-list', 'HEAD', '-n', '1', '--first-parent', '--before=2021-09-05'
   }):sub(1,-2)
 
-  if sha then
-    assert(vim.startswith(rev, sha), ('Plugin sha for %s does match %s != %s'):format(plugin, rev, sha))
-  end
+  -- if sha then
+  --   assert(vim.startswith(rev, sha), ('Plugin sha for %s does match %s != %s'):format(plugin, rev, sha))
+  -- end
 
   vim.fn.system{'git', '-C', plugin_dir2, 'checkout', rev}