Make the filename option, as exposed to the plugins, consistently relative to the working directory

[loganfsmyth]

Q	A
Fixed Issues?
Patch: Bug Fix?	Y
Major: Breaking Change?	N
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR
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.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/7992/

[rtsao]

I think it's worth noting that babel-loader seems to always pass an absolute path to thefilename option (I think this.resourcePath in the webpack loader API yields an absolute path).

This change may cause specialized plugins that only run in the context of babel-loader to break if they had assumptions about this. Of course, this wasn't part of the official public API, but it was an invariant for this use case, so that assumption is not entirely unreasonable.

[loganfsmyth]

I went back and forth about whether it made sense to default to relative or absolute and I think both are entirely possible.

I ended up going with relative, but in all honesty I'd be happy to change it if people feel it'd be easier to manage. I just want it to be consistent.

[KevinGrandon]

Consistency is awesome, so thanks for working on this. I think changing it to absolute would fix our use-case, as well as other folks who use babel-loader. It would be awesome if you're up for changing it to that. Happy to test out a beta/alpha version if it would help.

[loganfsmyth]

Alright, I've made a PR to make it absolute instead: #8044