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.
