fix(pack): update add() to have default load=false during startup

Problem: the `load=true` in `vim.pack.add()` means that `:packadd` is
  executed even during startup. This leads to force source of 'plugin/',
  which breaks the intended loading order (`:h load-plugins`) and
  results into sourcing them twice. This also makes it ignore
  `--noplugin` argument.

  Using `:packadd!` during startup is more appropriate, while `:packadd`
  afterwards is still more favorable to actually force 'plugin/' source
  (as there is no pre-defined mechanism that will load them later).

Solution: have `load=false` default during startup, `true` - afterwards.

runtime/doc/pack.txt

@@ -336,7 +336,7 @@ add({specs}, {opts})                                          *vim.pack.add()*
       • {opts}   (`table?`) A table with the following fields:
                  • {load}? (`boolean`) Load `plugin/` files and `ftdetect/`
                    scripts. If `false`, works like `:packadd!`. Default
-                   `true`.
+                   `false` during startup and `true` afterwards.
 
 del({names})                                                  *vim.pack.del()*
     Remove plugins from disk

runtime/lua/vim/pack.lua

@@ -669,7 +669,9 @@ end
 
 --- @class vim.pack.keyset.add
 --- @inlinedoc
---- @field load? boolean Load `plugin/` files and `ftdetect/` scripts. If `false`, works like `:packadd!`. Default `true`.
+--- Load `plugin/` files and `ftdetect/` scripts. If `false`, works like `:packadd!`.
+--- Default `false` during startup and `true` afterwards.
+--- @field load? boolean
 
 --- Add plugin to current session
 ---
@@ -693,7 +695,7 @@ 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 = true }, opts or {})
+  opts = vim.tbl_extend('force', { load = vim.v.vim_did_enter == 1 }, opts or {})
   vim.validate('opts', opts, 'table')
 
   --- @type vim.pack.Plug[]

test/functional/plugin/pack_spec.lua

@@ -137,10 +137,12 @@ function repos_setup.plugindirs()
 
   repo_write_file('plugindirs', 'lua/plugindirs.lua', 'return "plugindirs main"')
   repo_write_file('plugindirs', 'plugin/dirs.lua', 'vim.g._plugin = true')
+  repo_write_file('plugindirs', 'plugin/dirs_log.lua', '_G.DL = _G.DL or {}; DL[#DL+1] = "p"')
   repo_write_file('plugindirs', 'plugin/dirs.vim', 'let g:_plugin_vim=v:true')
   repo_write_file('plugindirs', 'plugin/sub/dirs.lua', 'vim.g._plugin_sub = true')
   repo_write_file('plugindirs', 'plugin/bad % name.lua', 'vim.g._plugin_bad = true')
   repo_write_file('plugindirs', 'after/plugin/dirs.lua', 'vim.g._after_plugin = true')
+  repo_write_file('plugindirs', 'after/plugin/dirs_log.lua', '_G.DL = _G.DL or {}; DL[#DL+1] = "a"')
   repo_write_file('plugindirs', 'after/plugin/dirs.vim', 'let g:_after_plugin_vim=v:true')
   repo_write_file('plugindirs', 'after/plugin/sub/dirs.lua', 'vim.g._after_plugin_sub = true')
   repo_write_file('plugindirs', 'after/plugin/bad % name.lua', 'vim.g._after_plugin_bad = true')
@@ -357,6 +359,45 @@ describe('vim.pack', function()
       eq(true, exec_lua('return pcall(require, "lspconfig")'))
     end)
 
+    describe('startup', function()
+      local init_lua = ''
+      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 })
+      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 validate_loaded = function()
+          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 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)')
+        validate_loaded()
+
+        -- Should only `:packadd!` already installed plugin
+        n.clear({ args_rm = { '-u' } })
+        validate_loaded()
+
+        -- Should not load plugins if `--noplugin`, only adjust 'runtimepath'
+        n.clear({ args = { '--noplugin' }, args_rm = { '-u' } })
+        eq('plugindirs main', exec_lua('return require("plugindirs")'))
+        eq({}, n.exec_lua('return { vim.g._plugin, vim.g._after_plugin }'))
+        eq(vim.NIL, n.exec_lua('return _G.DL'))
+      end)
+    end)
+
     it('shows progress report during installation', function()
       exec_lua(function()
         vim.pack.add({ repos_src.basic, repos_src.defbranch })