feat(pack): allow choosing update target in `update()`

Problem: There are two fairly common workflows that involve lockfile and
  can be made more straightforward in `vim.pack`:

    1. Revert latest update. Like if it introduced unwanted behavior.

    2. Update to a revision in the lockfile. Like if already updated
       on another machine, verified that everything works, `git add` +
       `git commit` + `git push` the config, and want to have the same
       plugin states on current machine.

Solution: Make `update` allow `opts.target`. By default it uses
  `version` from a plugin specification (like a regular "get new changes
  from source" workflow). But it also allows `"lockfile"` value to
  indicate that target revision after update should be taken from the
  current lockfile verbatim.

  With this, the workflows are:

    1. Revert (somehow) to the lockfile before the update, restart, and
       `vim.pack.update({ 'plugin' }, { target = 'lockfile' })`. If Git
       tracked, revert with `git checkout HEAD -- nvim-pack-lock.json`.
       For non-VCS tracked lockfile, the revisions can be taken from the
       log file. It would be nicer if `update()` would backup a lockfile
       before doing an update, but that might require discussions.

    2. `git pull` + `:restart` +
       `vim.pack.update(nil, { target = 'lockfile' })`.
       The only caveats are for new and deleted plugins:
        - New plugins (not present locally but present in the lockfile)
          will be installed at lockfile revision during restart.
        - Deleted plugins (present locally but not present in the
          lockfile) will still be present: both locally *and* in the
          lockfile. They can be located by
          `git diff -- nvim-pack-lock.json` and require manual
          `vim.pack.del({ 'old-plugin1', 'old-plugin2' })`.

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