Renaming stuff for `v1.0.0`

Sufficient for `1.0.0-rc.1` is renaming the functions, according to [the spreadsheet](https://docs.google.com/spreadsheets/d/1jT9k6ts7hXyQdhZSH4eAV8BwY1jSaUm233gHgWED7i4), without the module restructure.
Update comment. Should be "Get the selected root container."