These patches make Vim use native-search-paths to search for plugins,
instead of manually searching for them in a set of hardcoded paths,
which is how it was done up till now.
I've added the same search paths to Neovim, which unlike Vim, wasn't
auto-detecting Vim plugins at all before.
Neovim also searches XDG_DATA_DIRS for Neovim-specific plugins, so I'd
appreciate if https://issues.guix.gnu.org/48112 was merged too.
Finally, I've updated the package definitions to use the new style with
gexps and label-less inputs.