Auto-format imports

[japgolly]

This is part of #479. I'd love to do all of #479 on the 1.x branch but it would probably just make it harder to merge the outstanding PRs. Doing just the import side now though shouldn't be very disruptive and it lays a little more groundwork.

[armanbilge]

What does DCI=1 do?

[japgolly]

Facilitates this:

scala-js-dom/build.sbt

Lines 31 to 41 in b0fa8f9

	val inCI = Option(System.getenv("CI")).exists(_.contains("1"))
	val commonSettings = Seq(
	organization := "org.scala-js",
	scalacOptions ++= Seq(
	"-deprecation",
	"-feature",
	),
	scalacOptions ++= (if (!inCI) Seq.empty else Seq(
	"-Xfatal-warnings",
	)),

So -Xfatal-warnings isn't enabled locally, we need actual warnings so that scalafix can identify and remove unnecessary imports. Where as in CI, we expect that scalafix has already been run (as part of prePR) so we turn those warnings back into errors.

[armanbilge]

Wait a minute, doesn't -D set system properties and not environment variables?

[japgolly]

Oh I see you already approved, cool, merging

[armanbilge]

@japgolly Oops, maybe I should have revoked my approval until you clarified #518 (comment) 😅

[japgolly]

faaaaaaaaaaaark, good catch