It should be no different from Vim. Namely,
In nvim help :h vimrc describes how the associated file is (still) referred to as a "vimrc" (or "config file") and just below that (:h $MYVIMRC) you'll see how/when the environment variable $MYVIMRC is assigned, i.e. during startup nvim looks for config in four places (vs five ...
packpath, the option
packpath is an option, which means it can be altered using the set command. The set command already does de-duplication for appending values. For example,
It even adds/strips commas for you. It is not always possible, but if you can, use the ...
I have a file called ~/.config/nvim/after/ftplugin_after.vim
The correct path you're looking for is actually:
Note that ftplugin needs to be a directory, and that the *.vim file inside it must be named after the filetype of the files you would like to match.
The ftplugin scripts must always match a specific filetype, ...