Many filetypes from filetype.vim set buffer-local variables, meaning
vim.filetype.match cannot be used without side effects. Instead of
setting these buffer-local variables in the filetype detection functions
themselves, have vim.filetype.match return an optional function value
that, when called, sets these variables. This allows vim.filetype.match
to work without side effects.
In `filetype.lua`, source runtime `ftdetect` scripts within the `filetypedetect` augroup, same as `filetype.vim` (and only do so if `g:did_load_ftdetect` does not exist).
Filetype detection runs on BufRead and BufNewFile autocommands, both of
which can fire without an underlying buffer, so it's incorrect to use
<abuf> to determine the file path. Instead, match on <afile> and assume
that the buffer we're operating on is the current buffer. This is the
same assumption that filetype.vim makes, so it should be safe.
This default value is also set in filetype.vim, but if filetype.vim is
disabled the variable is never defined, which causes errors in some of
the dist#ft detection functions.
