Clean up warnings and scalac options.

[non]

This removes -deprecation warnings for 2.13 (since there are over 100
of them). It also tries to make it a bit easier to see which options
are used for which versions. Finally it removes an unused import.

I did this as an isolated PR because it's small and easy to review.

[non]

cc @dwijnand @SethTisue

I'm going to tag you all on these PRs that are primarily build-related. Let me know if anything jumps out at you.

[dwijnand]

LGTM. It's an interesting way to organise things, too.

[ashawley]

I worry this won't work for adding Dotty to the build. After #423 is merged, I predict people will relatively quickly request that Dotty be added as a compiler so there's no backsliding.

[non]

@ashawley I think the strategy will be easily to modify to support dotty. For example:

mk(211 to 214(...),
mk(211 to 213)(...),
mk(212 to 300)(...),
mk(214 to 301)(...))

(I think it's very unlikely we'll ever need more than 99 major versions, but we could always pad the 2/3 by 1000 if we needed to.)

In the worst case (where the ranges need to be disjoint) we can widen that type to IndexedSeq[Int], e.g. (211 to 213) :+ 300 (skipping 214).

[ashawley]

Dotty versions are of the scheme "0.17.0-RC1", see https://contributors.scala-lang.org/t/announcing-dotty-0-17-0-rc1/3556

[smarter]

Yeah, this will need to be changed for Dotty support. The usual way to do this sort of things is to pattern match on CrossVersion.partialVersion(scalaVersion.value): https://github.com/scala/scala-module-dependency-sample#depend-on-scala-modules-like-a-pro