| Q | A <!--(Can use an emoji 👍) -->
| ------------------------ | ---
| Fixed Issues? |
| Patch: Bug Fix? | Y
| Major: Breaking Change? | N
| Minor: New Feature? |
| Tests Added + Pass? | Yes
| Documentation PR | <!-- If so, add `[skip ci]` to your commit message to skip CI -->
| Any Dependency Changes? |
| License | MIT
Currently the `opts.filename` value exposed to plugins is just whatever the user passed. While it _could_ be relative to the working directory, if Babel was passed an absolute URL, it'll be absolute.
This PR explicitly ensures the filename is a relative path based on the working directory. This also exposes an officially endorsed API for reading the working directory path.
