Implement incremental compilation phases for `-Ybest-effort`

[jchyb]

Mostly relates to the sbt.ExtractDependencies phase, which is not run if any error is encountered before it.

To keep our system, where the scala 3 compiler developers do not have to worry about best effort features while fixing bugs (outside of an occasional addition of a best-effort test to a blacklist), the best way to implement this would be to add a best-effort only phase after typer which would make the trees coherent enough for ExtractDependencies to work, rather then changing anything in that phase or in Typer.