cabalfmt: use filepath arg instead of stdin

[namedots]

disclaimer: idk what I'm doing

my reasoning:
I wanted to let cabal-fmt auto-fill a field, but that didn't seem to be working due to current configuration feeding the content to cabal-fmt through stdin, so I wish to change it.
By default it seems neoformat uses a temporary path when passing file path by arg. That's also not good enough as cabal-fmt needs access to the project location.
I assume that the reason for using a temp path is that some tools make in-place changes. cabal-fmt only does so when asked to with -i, --in-place

does this break stuff for others?: I wouldn't know.

feel free to discard or whatever. I just made what seemed like an improvement to me, and figured out how to make the pull request.

[namedots]

oh. hold on. cabal-fmt would not receive changes in the buffer like this. that's no good.
and it doesn't do expansion when fed content on stdin.
and if doing it to a file in a temp path then it doesn't see the project and doesn't know how to expand.

meh.

[sbdchd]

Sounds good!

[namedots]

@sbdchd nonono hold up :<
it really should act on the buffer content, but my change throws that out of the window by instead supplying the file path.
no clue how I'm supposed to behave when I find problems after making the pull request.
still trying to figure this out.

[namedots]

I give up. I don't see how this could make sense. I'll close this and look for a corner to hide in. Maybe I managed to learn something at least.