Drop path when specifying out directory ?

When compiling file with explicit existing directory name (`app`) I would expect it to succeed.

```sh
cabal run -- agda2rust ./test/Hello.agda -o app
Agda/Primitive.rs
test/Hello.rs
app/test/Hello.rs: withFile: does not exist (No such file or
directory)
```

Perhaps `cabal run -- agda2rust ./test/Hello.agda -o app` should generate output in `app/Hello.rs` not `app/test/Hello.rs`