source lua files in load_plugin

[beardedsakimonkey]

(This is a recreation of #1030, which I accidentally closed by force pushing)

When installing packer from a clean slate[1], and running packer.sync() with config containing

use({"nvim-treesitter/nvim-treesitter", run = ":TSUpdate" })

I'd see the following error in my :messages (similar to #897 (comment)):

[packer.nvim] [ERROR 20:50:19] async.lua:20: Error in coroutine: ...ack/packer/start/packer.nvim/lua/packer/plugin_utils.lua:236: Vim:E492: Not an editor command: TSUpdate

I think this is because nvim-treesitter sets up the :TSUpdate command in plugin/nvim-treesitter.lua and packer only sources .vim files when loading a plugin.

After making the change to also source .lua files, I no longer see the error message.

[1] rm -rf ~/.local/share/nvim/site/pack/packer && rm ~/.config/nvim/plugin/packer_compiled.lua