Cross-build against dotty

[jisantuc]

Improvement

The biggest thing Scala 3 needs from the community is for everyone to begin porting their code. For established projects, the logical first step would be to start cross-building against the latest Scala 3 milestone, when they become available (or use a Dotty milestone right now!). We expect this to be a common scenario during the transition period. Or, you can jump straight in and upgrade your code to Scala 3 and try out some of the new features!

We’re especially interested in knowing if something blocks you completely, but other kinds of feedback are welcome as well.

Every Scala user can participate, but it’s especially helpful for maintainers of open-source libraries and tooling to begin moving. If you are a maintainer, then as an initial step, please consider adding Dotty to your project’s cross-build.

https://www.scala-lang.org/2019/12/18/road-to-scala-3.html#how-can-i-contribute

Notes + Context

We shouldn't fail CI if the Dotty build fails. This will be a smaller-scoped zone where we can monitor the upgrade situation with some of our commonly used dependencies.

[philvarner]

The long pole for this is likely to be spire (b/c of its extensive use of macros), which is a dependency of geotrellis-util, which is a dependency of geotrellis everything. typelevel/spire#936