I fixed many hidden bugs and made some small improvements. The main
reason this was separated from #336137 is to fix the sorting issue.
Before this commit, sorting for Vim plugins was broken and worked by
what was fetched first. This is because the sorting was done by empty
strings (the default value in CSV is not None but an empty string). This
PR fixes it and also moves sorting from the user to the library (from
`vim/plugins/update.py` to `pluginupdate.py`) to prevent such weird
issues and duplication of code.
* luarocks-packages-updater: convert into pyproject package
* pluginupdate: move to its own folder
so we can copy just the folder when using this as a module
* luarocks-packages-updater: adress review
