fix(pack)!: make default opts.load in add() to work inside 'plugin/'

Problem: Plain `vim.pack.add()` calls (with default `opts.load`) does
  not fully work if called inside 'plugin/' runtime directory. In
  particular, 'plugin/' files of newly added plugins are not sourced.
  This is because `opts.load` is `false` during the whole startup, which
  means `:packadd!` is used (modify 'runtimepath' but not force source
  newly added 'plugin/' files).

  This use case is common due to users organizing their config as
  separate files in '~/.config/nvim/plugin/'.

Solution: Use newly added `v:vim_did_init` to decide default `opts.load`
  value instead of `v:vim_did_enter`.

test/functional/plugin/pack_spec.lua

@@ -620,35 +620,42 @@ describe('vim.pack', function()
     end)
 
     describe('startup', function()
-      local init_lua = ''
+      local config_dir, pack_add_cmd = '', ''
+
       before_each(function()
-        init_lua = vim.fs.joinpath(fn.stdpath('config'), 'init.lua')
-        fn.mkdir(vim.fs.dirname(init_lua), 'p')
+        config_dir = fn.stdpath('config')
+        fn.mkdir(vim.fs.joinpath(config_dir, 'plugin'), 'p')
+
+        pack_add_cmd = ('vim.pack.add({ %s })'):format(vim.inspect(repos_src.plugindirs))
       end)
+
       after_each(function()
-        pcall(vim.fs.rm, init_lua, { force = true })
+        vim.fs.rm(config_dir, { recursive = true, force = true })
       end)
 
-      it('works in init.lua', function()
-        local pack_add_cmd = ('vim.pack.add({ %s })'):format(vim.inspect(repos_src.plugindirs))
-        fn.writefile({ pack_add_cmd, '_G.done = true' }, init_lua)
+      local function assert_loaded()
+        eq('plugindirs main', exec_lua('return require("plugindirs")'))
 
-        local function assert_loaded()
-          eq('plugindirs main', exec_lua('return require("plugindirs")'))
-
-          -- Should source 'plugin/' and 'after/plugin/' exactly once
-          eq({ true, true }, n.exec_lua('return { vim.g._plugin, vim.g._after_plugin }'))
-          eq({ 'p', 'a' }, n.exec_lua('return _G.DL'))
-        end
+        -- Should source 'plugin/' and 'after/plugin/' exactly once
+        eq({ true, true }, n.exec_lua('return { vim.g._plugin, vim.g._after_plugin }'))
+        eq({ 'p', 'a' }, n.exec_lua('return _G.DL'))
+      end
 
+      local function assert_works()
         -- Should auto-install but wait before executing code after it
         n.clear({ args_rm = { '-u' } })
         n.exec_lua('vim.wait(500, function() return _G.done end, 50)')
         assert_loaded()
 
-        -- Should only `:packadd!` already installed plugin
+        -- Should only `:packadd!`/`:packadd` already installed plugin
         n.clear({ args_rm = { '-u' } })
         assert_loaded()
+      end
+
+      it('works in init.lua', function()
+        local init_lua = vim.fs.joinpath(config_dir, 'init.lua')
+        fn.writefile({ pack_add_cmd, '_G.done = true' }, init_lua)
+        assert_works()
 
         -- Should not load plugins if `--noplugin`, only adjust 'runtimepath'
         n.clear({ args = { '--noplugin' }, args_rm = { '-u' } })
@@ -656,6 +663,13 @@ describe('vim.pack', function()
         eq({}, n.exec_lua('return { vim.g._plugin, vim.g._after_plugin }'))
         eq(vim.NIL, n.exec_lua('return _G.DL'))
       end)
+
+      it('works in plugin/', function()
+        local plugin_file = vim.fs.joinpath(config_dir, 'plugin', 'mine.lua')
+        fn.writefile({ pack_add_cmd, '_G.done = true' }, plugin_file)
+        -- Should source plugin's 'plugin/' files without explicit `load=true`
+        assert_works()
+      end)
     end)
 
     it('shows progress report during installation', function()