fix(pack)!: ensure plugin is fully absent if not fully installed

Problem: Currently it is possible to have plugin in a "partial install"
  state when `git clone` was successfull but `git checkout` was not.
  This was done to not checkout default branch by default in these
  situations (for security reasons).

  The problem is that it adds complexity when both dealing with lockfile
  (plugin's `rev` might be `nil`) and in how `src` and `version` are
  treated (wrong `src` - no plugin on disk; wrong `version` - "partial"
  plugin on disk).

Solution: Treat plugin as "installed" if both `git clone` and
  `git checkout` are successful, while ensuring that not installed
  plugins are not on disk and in lockfile.

  This also means that if in 'init.lua' there is a `vim.pack.add()` with
  bad `version`, for first install there will be an informative error
  about it BUT next session will also try to install it. The solution is
  the same - adjust `version` beforehand.

test/functional/plugin/pack_spec.lua

@@ -575,10 +575,9 @@ describe('vim.pack', function()
 
       local pluginerr_hash = git_get_hash('main', 'pluginerr')
       local ref_lockfile = {
-        -- Should be no entry for `repo_not_exist`
+        -- Should be no entry for `repo_not_exist` and `basic` as they did not
+        -- fully install
         plugins = {
-          -- No `rev` because there was no relevant checkout
-          basic = { src = repos_src.basic, version = "'not-exist'" },
           -- Error during sourcing 'plugin/' should not affect lockfile
           pluginerr = { rev = pluginerr_hash, src = repos_src.pluginerr, version = "'main'" },
         },
@@ -605,14 +604,12 @@ describe('vim.pack', function()
       eq(false, vim.tbl_contains(rtp, after_dir))
     end)
 
-    it('does not checkout on bad `version`', function()
+    it('does not install on bad `version`', function()
       local err = pcall_err(exec_lua, function()
         vim.pack.add({ { src = repos_src.basic, version = 'not-exist' } })
       end)
       matches('`not%-exist` is not a branch/tag/commit', err)
-      local plug_path = pack_get_plug_path('basic')
-      local entries = vim.iter(vim.fs.dir(plug_path)):totable()
-      eq({ { '.git', 'directory' } }, entries)
+      eq(false, pack_exists('basic'))
     end)
 
     it('allows changing `src` of installed plugin', function()