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`.

runtime/doc/pack.txt

@@ -379,7 +379,7 @@ add({specs}, {opts})                                          *vim.pack.add()*
                    Load `plugin/` files and `ftdetect/` scripts. If `false`,
                    works like `:packadd!`. If function, called with plugin
                    data and is fully responsible for loading plugin. Default
-                   `false` during startup and `true` afterwards.
+                   `false` during |init.lua| sourcing and `true` afterwards.
                  • {confirm}? (`boolean`) Whether to ask user to confirm
                    initial install. Default `true`.
 

runtime/lua/vim/pack.lua

@@ -743,8 +743,9 @@ local function pack_add(plug, load)
   -- NOTE: The `:packadd` specifically seems to not handle spaces in dir name
   vim.cmd.packadd({ vim.fn.escape(plug.spec.name, ' '), bang = not load, magic = { file = false } })
 
-  -- Execute 'after/' scripts if not during startup (when they will be sourced
-  -- automatically), as `:packadd` only sources plain 'plugin/' files.
+  -- The `:packadd` only sources plain 'plugin/' files. Execute 'after/' scripts
+  -- if not during startup (when they will be sourced later, even if
+  -- `vim.pack.add` is inside user's 'plugin/')
   -- See https://github.com/vim/vim/issues/15584
   -- Deliberately do so after executing all currently known 'plugin/' files.
   if vim.v.vim_did_enter == 1 and load then
@@ -760,7 +761,7 @@ end
 --- @inlinedoc
 --- Load `plugin/` files and `ftdetect/` scripts. If `false`, works like `:packadd!`.
 --- If function, called with plugin data and is fully responsible for loading plugin.
---- Default `false` during startup and `true` afterwards.
+--- Default `false` during |init.lua| sourcing and `true` afterwards.
 --- @field load? boolean|fun(plug_data: {spec: vim.pack.Spec, path: string})
 ---
 --- @field confirm? boolean Whether to ask user to confirm initial install. Default `true`.
@@ -788,7 +789,7 @@ end
 --- @param opts? vim.pack.keyset.add
 function M.add(specs, opts)
   vim.validate('specs', specs, vim.islist, false, 'list')
-  opts = vim.tbl_extend('force', { load = vim.v.vim_did_enter == 1, confirm = true }, opts or {})
+  opts = vim.tbl_extend('force', { load = vim.v.vim_did_init == 1, confirm = true }, opts or {})
   vim.validate('opts', opts, 'table')
 
   local plug_dir = get_plug_dir()

test/functional/plugin/pack_spec.lua

@@ -620,18 +620,18 @@ 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')
-      end)
-      after_each(function()
-        pcall(vim.fs.rm, init_lua, { force = true })
+        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)
 
-      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)
+      after_each(function()
+        vim.fs.rm(config_dir, { recursive = true, force = true })
+      end)
 
       local function assert_loaded()
         eq('plugindirs main', exec_lua('return require("plugindirs")'))
@@ -641,14 +641,21 @@ describe('vim.pack', function()
         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()