toVimPlugin: prepends 'vimPlugin' to the name

Neovim plugins built with buildNeovimPlugin are based on derivations
from the lua package set, ie., they are a flat-install (no share/lua/5.1
nesting) with some post-processing.

While debugging, I noticed their name lacked the vimplugin prefix,
making them a bit harder to diagnose if the derivation was the vimplugin
or not.
So I hardcoded 'namePrefix' (why let the user change that ?) and moved
it to toVimPlugin so it applies to lua-based plugins as well.

Before this PR:
➜ nix-build -A vimPlugins.plenary-nvim
/nix/store/zyvdf7c1k4q2ykg6jydpf5c31g9j921s-lua5.1-plenary.nvim-scm-1-unstable-2024-03-25

After this PR
➜ nix-build -A vimPlugins.plenary-nvim
/nix/store/dy4rjzjbhshi109i6969f39vd9xbjgs1-vimplugin-lua5.1-plenary.nvim-scm-1-unstable-2024-03-25

pkgs/applications/editors/vim/plugins/build-vim-plugin.nix

···
 
   buildVimPlugin =
     { name ? "${attrs.pname}-${attrs.version}"
-    , namePrefix ? "vimplugin-"
     , src
     , unpackPhase ? ""
     , configurePhase ? ":"
···
     }@attrs:
     let
       drv = stdenv.mkDerivation (attrs // {
-        name = namePrefix + name;
+        name = lib.warnIf (attrs ? vimprefix) "The 'vimprefix' is now hardcoded in toVimPlugin" name;
 
         inherit unpackPhase configurePhase buildPhase addonInfo preInstall postInstall;
 

pkgs/applications/editors/vim/plugins/vim-utils.nix

···
 
   toVimPlugin = drv:
     drv.overrideAttrs(oldAttrs: {
+      name = "vimplugin-${oldAttrs.name}";
       # dont move the "doc" folder since vim expects it
       forceShare = [ "man" "info" ];