Syntax loaded incorrectly/twice with lazy loaded filetype plugin

When a plugin has a `after/syntax/lua.vim` file, and this plugin gets loaded `'for': ['lua']`, the main syntax plugin(s) (`syntax/lua.vim`) will not be loaded, when `b:current_syntax` might exist already.

In the case of https://github.com/raymond-w-ko/vim-lua-indent/blob/master/after/syntax/lua.vim#L7 this relies on a variable `lua_version` being set, but it gets not set from Vim's `syntax/lua.vim` when`b:current_syntax` is set already.

TEST CASE:
1. Create a `config.ld` file, it will have `ft=ld` and `b:current_syntax='ld'`
2. `set ft=lua` (manually or through a modeline)

vim-plug will call `runtime syntax/lua.vim`, but that finishes, because `b:current_syntax` is set to `'ld'` already.
Then `after/syntax/lua.vim` from the plugin gets sourced, and throws an error because of the unknown var.

I've noticed that passing in `, syn, 'after/'.syn` to `s:lod` will even cause it to be loaded twice in the end?!

So the following would actually fix it for this use case, but breaks the "Filetype-based on-demand loading" test (`'xxx/syntax', 'xxx/after/syntax'` is missing from `g:xxx`).

```diff
diff --git i/plug.vim w/plug.vim
index 95d8eab..a0d6b1c 100644
--- i/plug.vim
+++ w/plug.vim
@@ -489,7 +489,7 @@ endfunction

function! s:lod_ft(pat, names)
let syn = 'syntax/'.a:pat.'.vim'
- call s:lod(a:names, ['plugin', 'after/plugin'], syn, 'after/'.syn)
+ call s:lod(a:names, ['plugin', 'after/plugin'])
execute 'autocmd! PlugLOD FileType' a:pat
call s:doautocmd('filetypeplugin', 'FileType')
call s:doautocmd('filetypeindent', 'FileType')
```

When testing it manually, the `syntaxset` FileType event will trigger the syntax files to be loaded automatically, but for some reason that is not happening during the tests, although the autocommand is there?!
```
syntaxset FileType
* exe "set syntax=" . expand("")
```

A test case (`t/minimal.vim`):
```
set rtp+=../$PWD

call plug#begin('~/.vim/plugged')
exe "Plug '".expand("$PWD")."/p1', {'for': 'lua'}"
" Plug 'raymond-w-ko/vim-lua-indent', {'for': 'lua'}
call plug#end()
```

With `p1/after/syntax/lua.vim`:
```vim
echom lua_version
```

Then run it with `vim -Nu minimal.vim t.ld` and call `set ft=lua` in Vim:
```
Error detected while processing /home/user/Vcs/vim-plug/t/p1/after/syntax/lua.vim:
line 1:
E121: Undefined variable: lua_version
E15: Invalid expression: lua_version
5
```

A fix for this might be to `unlet! b:current_syntax` in `s:lod_ft`, but then it will (still) cause the syntax files to be sourced twice - which seems to be wrong in the first place?!