PR #8418 introduced some new logic that uses the third parameter of `fs-readdir-recursive`'s `filter` option, the current directory, but missed that our dependencies only required `^1.0.0`, while that parameter was actually added in `1.1.0`. This means that if some other dependency forced a downgrade in the version to satisfy some range, or the user already had 1.0.0 module installed locally, it could cause problems,
* Add sourcemap examples for identifiers in code generator.
* Generate sourcemaps with exact mappings for identifiers, even if leading/trailing tokens also use that location.
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Once merged I will make the corresponding documentation changes on the website (in a 7 branch)
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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.
