auto-detect elm version per file, instead of based on cwd

In 0.8.1, the elm-version is auto-detected based on the presence of `elm.json` or `elm-package.json` in the current working directory.

Proposal: instead of checking the current working directory, for each file check the file's folder and if not found traverse upwards until a package file is found.

It seems like this would be more flexible, and still do the right thing in all current cases? I think this would also allow plugins to not have to pass `--elm-version` unless they are using `--stdin`

Start at the elm-version auto-detection entry point and trace how input files, --stdin, and the current working directory are handled. Check the lookup behavior for elm.json and elm-package.json, then verify that each file searches from its own folder upward and that existing cases still work.

Written by the indexing model from the issue text.