g:mkdp_command_for_global required unless Vundle

If I install with `vundle`, things work as expected. But if I install with `minpac` or natively (see https://github.com/iamcco/markdown-preview.nvim/issues/56) I need to include `let g:mkdp_command_for_global = 1` or else I can't do any previews.

After installing via `minpac`, here are steps to reproduce:

```
vi test.md
:MarkdownPreview

E492: Not an editor command: MarkdownPreview
```

If I add `let g:mkdp_command_for_global = 1` to `.vimrc` then it works to preview my `test.md` file. When installed via `vundle` I don't need that config and I can't figure out why

```
$ vim --version | head -n 3
VIM - Vi IMproved 8.2 (2019 Dec 12, compiled May 8 2021 05:44:12)
macOS version
Included patches: 1-2029
```